After all, if anyone were paying attention to either trip (which does not seem to be happening, anyway), Cheney's surprise visit would seem to undermine the newsworthyness of McCain's planned trip.
What is more, as McCain seeks to be his own man -- in the pubic's mind, at least, this further ties McCain not just to Bush -- but to someone perhaps even less popular: Dick Cheney.
... This is sort of like when your parents announce they are coming with you on your big trip with your buddies (see Joe Lieberman, Lindsay Graham, et al.). You can't exactly say "no," but you ask them to drop you off a few blocks away before you meet up with your friends.
Look, I know this is a serious trip. But I don't see how the timing of this is helpful to McCain ...