Is the tree of "knowledge of good and evil" the same as the "tree of life"? 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.


  The LORD God made all kinds of trees grow out of the ground--trees that were pleasing to the eye and good for food. In the middle of the garden were the tree of life and the tree of the knowledge of good and evil. (Gen 2:9)


At first, only one was made off limits to humans.


  but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die. (Gen 2:17)


Then, after humans disobeyed, God made the other tree off limits, as well. 


  And the LORD God said, "The man has now become like one of us, knowing good and evil. He must not be allowed to reach out his hand and take also from the tree of life and eat, and live forever. (Gen 3:22)


But now... it is under guard.


  After he drove the man out, he placed on the east side of the Garden of Eden cherubim and a flaming sword flashing back and forth to guard the way to the tree of life. (Gen 3:24)


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.)