An intersting theory which had been in existence is the two seed theory which is not commonly accepted by churches because of their inherent difficulties.  However as a cogent coherent theory it needs to be mentioned. In its logical stand and biblical support it will stand at par with Calvinism, though only a few have  really studied it.

ADAM, EVE AND THE SERPENT

It starts with the persons of Eve, Adam and the Serpent who were involved in the drama. Serpent is mentioned in the Bible as
Gen 3:1  Now the serpent was more crafty than any of the wild animals the LORD God had made.
Rev 20:2 ....the dragon, that ancient serpent, who is the devil, or Satan,....
So there is reason to assume that this serpent was Satan who was hurled down to earth after the fall of the angels.
Most Chrisitans do identify the serpent as either Satan himself or the creature serpent possessed by Satan.  The later curse on serpent indicates that the serpent was not at that time crawling on its belly. Serpent is identified in Revelation as
Rev. 12:9  The great dragon was hurled down--that ancient serpent called the devil, or Satan, who leads the whole world astray. He was hurled to the earth, and his angels with him.
The  other references to Satan also corroborates it thus:
Ezek 28:12 "…Thou sealest up the sum, full of wisdom, and perfect in beauty."
Ezek 28:13-14 "Thou hast been in Eden the garden of God…
The context in which the story starts is interesting
Gen 2:22  Then the LORD God made a woman from the rib he had taken out of the man, and he brought her to the man.
23  The man said, "This is now bone of my bones and flesh of my flesh; she shall be called 'woman, ' for she was taken out of man."
24  For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh.
25  The man and his wife were both naked, and they felt no shame.
It is a direct context of marriage and sexual relations within the marriage when the husband and wife were naked and were not ashamed.

KUNDALINI YOGA

The tree of the knowledge of good and evil is certainly a symbolism.  Tree in its basic hebrew means fixed or rooted.,
Knowledge (da'ath  Strong#1847) means cunning, wittingly knowing, Good (towb Stong#2896) means beautiful, leasant) and Evil (ra Strong#7451) means wickedness, displeasure, affliction.  The fruit of the tree of the Knowledge of Good and Evil to which this fallen Satan led Eve (who was an innocent girl) was the tantric art of sex.  The Kundalini Yoga as known in the East details this art.

It is directly related to a symbolism of coiled serpent.  With this in mind read the following verses.

Gen 3:1  Now the serpent was more crafty than any of the wild animals the LORD God had made. He said to the woman, "Did God really say, 'You must not eat from any tree in the garden'?"
2  The woman said to the serpent, "We may eat fruit from the trees in the garden,
3  but God did say, 'You must not eat fruit from the tree that is in the middle of the garden, and you must not touch it, or you will die.'"
4  "You will not surely die," the serpent said to the woman.
5  "For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil."
6  When the woman saw that the fruit of the tree was good for food and pleasing to the eye, and also desirable for gaining wisdom, she took some and ate it. She also gave some to her husband, who was with her, and he ate it.
This art of Kundalini Eve taught to Adam also. The result was shame and feeling of nakedness.
 Dangers of Kundalini Yoga
Here is what the serpent said
Kundalini in Modern revivals
7  Then the eyes of both of them were opened, and they realized they were naked; so they sewed fig leaves together and made coverings for themselves.
8  Then the man and his wife heard the sound of the LORD God as he was walking in the garden in the cool of the day, and they hid from the LORD God among the trees of the garden.
9  But the LORD God called to the man, "Where are you?
10  He answered, "I heard you in the garden, and I was afraid because I was naked; so I hid."
11  And he said, "Who told you that you were naked? Have you eaten from the tree that I commanded you not to eat from?"
12  The man said, "The woman you put here with me--she gave me some fruit from the tree, and I ate it."
13  Then the LORD God said to the woman, "What is this you have done?" The woman said, "The serpent deceived me, and I ate."
14  So the LORD God said to the serpent, "Because you have done this, "Cursed are you above all the livestock and all the wild animals! You will crawl on your belly and you will eat dust all the days of your life.
Introduction of sex in the life cycle was therefore considered as the Original sin.  This therefore transmitted genetically the codes of the parents onto the children.  Thus in Adam all sinned and in Adam all died.

There is not unanimity among the proponents whether sex was intended by God as a reproduction process or not.  The command to multiply came before the fall.  Hence it is to be understood that sex was divinely instituted from the beginning.  The sin was not in the sex act itself.  It was in the purpose for which this yoga was perfomed.  If you have read through the Kundlaini Yoga, the avowed aim of the yoga is to attain Godhead - to be like God.  While Man is an image of God, the Yoga tries to make use of the similarity to attain the other dimension by a magical performance of the humane dimension.

The two seed theory however does not go into this aspect but goes on the purpose of Satan in thwarting God's plan of salvation by planting his seed into the woman.

The question remains whether this sex act of Satan with Eve was spiritual or physical.  Many argue that Satan who was a spirit  could not have had sex with Eve who was human. This is based on the assumption that Angels are spirit - of which Satan was one before his fall.  But they do have bodies, even though they have other spiritual powers.  The word Angel does not mean spirit beings, but simply messenger.

THE NEPHILIMS
In this connection we have the reference to Sons of God who went into Daughters of men in Gen 6.  While there are debates over who this Sons of God are, one assumption is that they were aliens from outer space - angels.  This is corroborated by a detailed description of this in the Book of Enoch.  The existence and the validity of the Book of Enoch is found in:
Jude 1:6  And the angels who did not keep their positions of authority but abandoned their own home--these he has kept in darkness, bound with everlasting chains for judgment on the great Day.
I had so many enquiries about this that I include the Book of Enoch as a reference.  This book though not canonical explains how angels saw the beauty of the women of earth and came down and married them.
Gen 6:4  The Nephilim were on the earth in those days--and also afterward--when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown.
The word Nephilim (#5303) comes  from the root 'naphal' (#5307) meaning to fall away, outcast ones indicating the orign of fallen angels. Others take it from its root nephil to mean "that which breathes" or "embodied" being

If these were indeed alien angels, then Satan also could have sex either as an embodied being or in possession of the animal Serpent. Satan did not approach Adam  but Eve because men don't bear "fruit". Satan could corrupt mankind spiritually and  physically through Eve.
 Thus the father of Cain could hav been  Satan and Father of Adam was Abel.  Cain killed Abel to destroy the line of Adam and take over the earth.  But Seth was born to carry the name of Adam.
Gen 3:14-15 "And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life: (v. 15) And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel."
Thy seed here refers to the children of Satan through Eve and her seed is of Adamic origin.

 Gen 4:1-2 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD. (v.2) And she again ((#3254) yacap)bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.(4:1  Adam lay with his wife Eve, and she became pregnant and gave birth to Cain. She said, "With the help of the LORD I have brought forth a man."
2  Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil.NIV)

Indicates Cain and Abel were twins.  But they were from two different fathers.
 I Jn 3: Jn 3:8-10 "He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil. (v.9) Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God. (v.10) In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother." 12 Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous.

KENITES
The descendants of Cain are the Kenite (which means sons of Qayin) probably mixing with Cananites.

Gen 4:15-16 "And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him. (v.16) And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden."
Two Genealogies
Genesis 4:17-24 Cain knew his wife, and she conceived and bore Enoch; and he built a city and called the name of the city after the name of his son, Enoch. To Enoch was born Irad; and Irad was the father of Mehujael; and Mehujael was the father of Methushael, and Methushael the father of Lamech. .....
Genesis 5:1 This is the book of the generations of Adam. In the day that God created Adam, in the likeness of God he made him. He created them male and female and blessed them, and called their name Adam, in the day that they were created. And Adam lived one hundred and thirty years and became the father of a son in his own likeness, after his image, and named him Seth.
Cain was still alive and if he had been Adam's son would have been in his likeness and after his image, but he was not Adam's son; he was the son of Satan and that  is why he is not listed in Adam's geneology.
But at the time of Noah, did not god destroy the whole mankind.  This is a a difficult situation.  Two solutions are given for this.  First if we assume that the flood was only local.  Kenites were saved because some of them lived far away from the plains.  An alternate explanation is that the Fallen Angels took them with thm into their space vehicles.  The  Cananite tradition is that the survivors were taken up into the heavens.  So  God saved Noah by the ark, but the angels tricked again and saved their progeny by taking them up till the floods were abated.

THE ELECTION
Cain's descendant's, the Kenites, became servants and then scribes to the Levitical priests, whose duty it was to teach the law and keep a written record which we now have as the Holy Bible.
1 Chr 2:55 "And the families of the scribes which dwelt at Jabez; the Tirathites, the Shimeathites, and Suchathites. These are the Kenites that came of Hemath, the father of the house of Rechab."
Because of this they falsified the word of God.
Jeremiah 8:8 How can you say, "We are wise, and the law of the Lord is with us?" But, behold, the false pen of the scribes has made it into a lie.
By the time we come to the time of Jesus they have taken over the control of the temple and the law was misinterpreted apart from its spirit, just as Satan did with Eve.  So Jesus tells the Pharisees and the Saducees:
Jn 8:43  Why is my language not clear to you?  ...........
44  You belong to your father, the devil, and you want to carry out your father's desire. He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies.
Remember the first murderer was Cain. The Kenites took over Israel.  The true jews were blinded.
In the parable of the tares, Jesus identifies the two seeds.
Now we come to identify the elect and the damned.  The elect are predestined to glory  and non-elect to burning fire.  It was predestined.  There was election, the atonement of Jesus was indeed limited for the elect and there is grace which is irresistible for the elect and elect only.  Hear it from the mouth of Jesus himself.:
Mat 13:24  Jesus told them another parable: "The kingdom of heaven is like a man who sowed good seed in his field.
25  But while everyone was sleeping, his enemy came and sowed weeds among the wheat, and went away.
Was he talking about Adam and Eve?
26  When the wheat sprouted and formed heads, then the weeds also appeared.
27  "The owner's servants came to him and said, 'Sir, didn't you sow good seed in your field? Where then did the weeds come from?'
28  "'An enemy did this,' he replied. "The servants asked him, 'Do you want us to go and pull them up?'
29  "'No,' he answered, 'because while you are pulling the weeds, you may root up the wheat with them.
The wheat and the tares are so mingled it is difficult to seperate them.  So they will remain till the end of the Age.
30  Let both grow together until the harvest. At that time I will tell the harvesters: First collect the weeds and tie them in bundles to be burned; then gather the wheat and bring it into my barn.'"
37  He answered, "The one who sowed the good seed is the Son of Man.
Adam and now Christ.
38  The field is the world, and the good seed stands for the sons of the kingdom. The weeds are the sons of the evil one,
39  and the enemy who sows them is the devil. The harvest is the end of the age, and the harvesters are angels.
40  "As the weeds are pulled up and burned in the fire, so it will be at the end of the age.
41  The Son of Man will send out his angels, and they will weed out of his kingdom everything that causes sin and all who do evil.
42  They will throw them into the fiery furnace, where there will be weeping and gnashing of teeth.