Viewing information on multiple accounts
SE does not make any information about users public other than what they decided to share themselves (with the exception of suspension notices, but those don't give much information). So, you can only know about the coexistence of accounts if the user shares this information, like some people do on other SE sites: e.g., Shadow of the Shadow and NotDoorknob.
SE position (as understood by me)
While cross-voting is definitely prohibited, this is not the only disallowed behaviour for multiple accounts. The guide for moderators mentions "supporting own position" as one non-reputation related form of abuse, and it says that the list is not exhaustive. There is a gray area as in many other places, something that is OK as a singular event may be not OK if it becomes a pattern. I quote two SE employees:
"Faking it" also includes pretending to be two people where the content is concerned. Don't support yourself in comments, don't answer your own questions, don't stir up staged controversy (or quell it by playing a supportive peer) -- Robert Cartaino
So long as there's no voting or other dubious-looking stuff (like one account asking a question and another immediately answering it) going on between the accounts, it's really not a big deal. -- Anna Lear
Note the parenthetical.
Answering with a pre-cooked answer
In theory this should not matter as long as accounts don't vote up or accept each other's posts. According to this theory, any upvotes received for an answer signify that it was also helpful to the voter, so the whole Q&A is a beneficial addition to the site.
In practice, specifically on this site, answers are sometimes upvoted in appreciation of the answerer being helpful to the asker, even if the voter already knew everything that's in the answer. There is a good chance that the voters would not approve of the Q&A pair if they knew it's a setup, especially if it's a low-level problem with near-duplicates already existing on the site.
My suggestion: vote up only when you personally benefited from the post, or feel it is a worthy contribution to the site as a knowledge base. If it's something of use only to the OP, let the OP reward it: they can at least accept the answer, even when they can't vote. I think there's been enough of "+1 for being nice" already.