If their naming is anything to go by then it’s not possible they were twins.
“I will put enmity between you and the woman, and between your offspring and her offspring; he shall bruise your head, and you shall bruise his heel."”
Eve listening to the pronouncement made upon the deceiver understood that a child, a male child will crush the head of the deceiver and this would, at least in her mind mean regaining access to the lost paradise.
So when she had a child and saw it was a male child she exclaims
“Now Adam knew Eve his wife, and she conceived and bore Cain, saying, "I have gotten a man with the help of the Lord."”
That’s what the English says but the Hebrew says אישׁ את־יהוה which is roughly, she acquired the man-god or the god man or the Messiah. This she imagined would be that deliverer she anticipated. Except for the fact that this child was born of Adam and not the seed of the woman exclusively like Jesus which is what Genesis 3:15 prophecies precisely.
Seeing she did not get her deliverer she named the next child vapor out of disappointment because her dreams all vanished like a vapor. This would give reasons to believe they were born apart in age and not twins.
As for the eth, there are other twins in the Bible and they don’t follow the proposed explanation. Esau and Jacob have eth but Esau alone has it first as the descendant line bearer however when he gives up his right to first born it switches to Jacob henceforth.
If anything, Cain and Abel were both possible line bearers for the promised Messiah except Cain loses the eth when he sins and he never has the eth next to his name again and Abel is murdered without descendants. Seth takes up the eth v25 as the line bearer for the seed through which the Messiah would come.