well, there are two different combos that I would be interested in.
Modern style combo: Beretta 92F and Beretta CX4 Storm.
not just ammo compatibility, but magazine compatibility, too. also, 9mm is cheap, and I've got loads and loads of it lying around. not great for really long shots (more than 150yds, I would think), but it would be great in an urban setting, and, of course, cheap fun at the range. only requires purchasing one gun, since I already have the 92
Classic combo: Marlin lever action in .357mag or .45Colt, and (for my tastes) an S&W Double action in the same caliber.
requires purchase of both guns, since I have neither. not as quick to reload. cartridge is much more powerful, however. ammo is more expensive. overall purchase price of the firearms themselves would be cheaper, methinks, though a bit more expensive to feed, especially in .45Colt.