One is the tree of life, the other is the tree of death. One might wonder why God would put the tree of life in the garden in the first place if He was going to prevent man from taking the fruit of it.
I think part of the answer lies in the fact that God gives us choices between right and wrong, light and darkness. To put a tree of death in the garden, there must also be a tree of life. To put a tree of life in the garden, there must also be a tree of death. God gives us choices that we make of our own free will.
The fact that God prevented man from getting to the tree of life is a clear indication that God has given us a free will to choose. We may choose the tree of life (Christ) or we may choose the tree of death (everything but Christ).
By the way, if Adam and Eve had partaken of the tree of life, it would have meant being in this body of sin forever. God preventing that is His mercy and grace!
God wants us to live forever, but in a glorified body and existence that is without sin and all its miseries.