Is there one off-limits tree in the Garden of Eden or two? Genesis speaks of the tree of knowledge of good and evil (Genesis 2:17) and then later talks of the tree of life (Genesis 3:22). Are they the same tree or two different trees?
No. These are two different trees.
At first, only one was made off limits to humans.
Then, after humans disobeyed, God made the other tree off limits, as well.
But now... it is under guard.
In summary: There are two trees (among many) in this story, the fruit of one was forbidden, humans disobeyed God. Now we are locked out of the other tree (from which the humans were allowed to eat in the first place.)
The tree of knowledge of good and evil is forbidden. The tree of life sustains life. Because, Adam and Eve disobeyed God, their right to eat from this tree was denied. However, the right to eat from this tree will be restored.
Revelation 22:14 KJV
Genesis 2:17 says that God placed a Tree of the Knowledge of Good and Evil in the middle of the Garden of Eden, telling Adam that he must not eat the fruit of this tree. By way of explanation, the ancients thought of existence as comprising good and evil, so knowing both good and evil meant to know everything.
Leon R. Kass says, in The Beginning of Wisdom: Reading Genesis, page 69, that the Tree of Life had not been a concern, because until Adam and Eve ate from the Tree of Knowledge, they were unaware that they were not immortal. As long as they had not eaten the fruit of the Tree of Knowledge, God had no need to forbid Adam from eating the fruit of the Tree of Life that he had also planted in the Garden of Eden. So that the people would not now eat of the Tree of Life and thus become immortal, God sent Adam and Eve from the Garden and placed cherubim (Hebrew; sing: cherub) and a flaming sword to keep (block) the path to the Tree of Life (Genesis 3:24).
Kass believes the Tree of Life was actually more important than the Tree of Knowledge. He says that once Adam and Eve are aware of their mortality, immortality becomes at once a conscious desire and a known impossibility. By placing a tree of life in our mythical original condition, and by showing original man's indifference to it, the Garden of Eden story speaks more to the impossible longings of its readers than of the desires of innocent man. Indeed, the Bible may even regard the human longing for (literal) deathlessness as mistaken, and limitless life as undesirable for a creature such as man.