You can tell if you use Yahoo StatTracker or the MLB Gamecast java applet thing, which gives you lineups like 30 mins before gametime, which lets you go and change your lineup before the games start. The problem with that is that if your backup catcher plays on the east coast and top catcher plays on the west coast, you won't be able to change it, since the backup will have already played. That's why getting your catcher's real life backup could be a plan.
Also, when a catcher starts a night game and then has a day game the next day, 99% of the time, the manager will play it safe and bench him to avoid excessive strain. That said, I still don't think carrying extra catchers is the best use of roster space. I'd rather have another bench player or have a starting pitcher.