Yes, answers from two different questions can be merged.
The answers from one question are migrated to the other, and former is closed as a duplicate of the latter.
However, this is only appropriate if they are exact duplicates: that is, if every answer to the first question is an appropriate answer to the second. Often we close A as a duplicate of B because the answers to B happen to contain information about A, but A and B are sufficiently different that answers to each should be different.
This is a moderator-only feature, so if you think two questions should be merged, you should raise a custom flag on one of the questions and explain exactly how the merge should happen.
Quoting from Jeff Atwood, Improved Question Merging:
Our kinder, gentler question merge has none of these problems! It behaves much more like a typical duplicate close, with the exception that the answers are migrated to the target question. [...]
If you believe [two questions] should be merged, flag them for moderator attention and indicate what you believe the merge target should be. (Yes, merging is still a moderator only function at the moment.) If the moderator agrees, the merge will happen.
(I believe that blog post describes the current implementation. If not, it's very similar. Example of two recently merged questions from SFF, which had the unusual side-effect that a question was answered nine months before it was asked: https://scifi.meta.stackexchange.com/q/6500/3567)