Had they already eaten from the Tree of Life, Jehovah driving them out of the garden would have been pointless, because they would have achieved immortality.
Not to put too fine an edge on this - but are we really sure that just eating from the Tree of Life ONCE would have given them everlasting life?
If so, how much? Just one bite? A whole fruit? Three or four fruits?
Or, would they have to have eaten from the tree of life every day? Or, every week or so?
Does Genesis actually say that they had never eaten from the tree of life before they were cast out?
Why was the tree of life destroyed in the flood? Apparantly, every other species of tree survived being immersed for all that time...
These and other such thoughts should make it clear that this whole story is ancient metaphor.