I never used to play more than one at a time, but now I'm playing both FF1:DoS and FF:X-2 at the same tie. X-2 is my "main-game" at home, while I play DoS on the bus to and from my job.

You could also count FF:CC as a third game as we play it multiplayer now and then, though we focus on Zelda: Four Swords Adventure at the moment.