What if they both ate from the tree of life first and then the tree of knowledge? How could they live forever if when sin entered the world death did also?
My opinion.
The 'tree of life' always brings up many questions to me. For example, why is a tree of life necessary to a pair of sinless humans? Just what would the furit of such a tree provide towards eternal life? (Gen. 3:22)
We see the 'tree of life' again in the New Heaven and Earth. (Rev. 22:2) Why, since sin has been done away with and all have resurrected bodies? Yet it's for the healing of the nations. Why, since there should be no sickness or pain anymore?
Somethings to consider concerning your questions.
In all likelihood, Adam and Eve did eat of the Tree of Life before their fall. It was not off limits. Only the 'tree of knowledge of good and evil' was off limits. We are not told that they did or didn't eat of it before the fall.
Understand that 'death' is not annihilation. It is 'separation'. Spiritual death is the separation of our spirit from God. Physical death is the separation of our spirit from our body. When Adam and Eve sinned, they died, in that they were now separated from God. And death was working now in their bodies, slowly destroying it so that one day it would cease to exist.
When God said, 'lest he put forth his hand, and take also of the tree of life, and eat, and live forever' certainly shows the power God had given in the fruit of that tree. It had the power to keep the body from it's eventual destruction and separation from the spirit. But it didn't have the power to deliver fallen man from his fallen state. And that is a condition God would not consider.
I think we must also consider the doctrine of 'election' to your questions. Again, my opinion.
Adam and Eve were children of God. They would always be children of God. God always planned on His children being born of them. The fall did not change that. The fall caused their separated condition from God. The fall introduced others being born of them who were not children of God. The fall required God to provide a remedy to redeem His children.
All born of Adam live forever somwhere. With God or without God. When God references 'living forever' He is speaking of living forever in an unhindered relationship with Him. Apparently, had Adam and Eve ate of the 'tree of life' they would have lived forever, and perhaps their children would also, in that state.
I'm not being dogmatic. Just thinking through this. I know many questions surround it.
Lees