Unfortunately the only way to play as more than one team is to set up 32 users (individually, sigh) and have the team you want to use be the active team. And even with 32 users you can sim games individually or play a game by selecting on the schedule. Neither can you select the unis for both teams (home wearing white early in the season leaves the visitor still in white) or change teams in-game to change the DC or play a game running both offenses. Might be able to do it with 2 controllers but I haven't tried that.
Oh, and if you go 32 users and have game prep set to CPU for the other 31 teams, you'll get freezes.