See the sticky thread at the top of the forum for a couple links to threads discussing different methods for mixing two sources.
Before you put the effort in, though, think about what you're trying to get out of the mix and whether it's likely to be any better than the 2-ch recordings. I find most 2-ch recordings better than 2 x 2ch recordings, with occasional exceptions in situations where the taper decides up front to mix all four channels and adjusts technique and configuration accordingly to achieve a specific desired result (and even then I think it often doesn't work out).