This may be a moot point because the Guitars.SE merge could be happening really soon but I have seen a couple of duplicate questions that we already answered over on guitars on Music.SE already. Is it acceptable to simply redirect the user to the already answered question? How do we handle this scenario?
I would not redirect a user who asks an on-topic question to go to a different site. Remember that it's OK to duplicate content; if it's an exact duplicate, then one will simply be closed and point to the other when the sites are merged. The answers can even be moved from one question to the other if the mods think it is warranted.
As for answering the question, I would go ahead. If the answer from Guitars is good, copy it and reference it. Make your answer Community Wiki if you don't feel comfortable receiving rep for "stealing" an answer.