Because both my babies cuddle so much, and the carrier is so huge, I once tried to see if I could have both of them in the carrier at the same time.
If it didn't work I'd have to take both of them to the vet seperately.
BUt they seemed to really enjoy it... or at least took comfort in the fact that the other's friend is there.
So I can take both Clover and Echo to the vet at the same time, because even in one crate (for short periods of time, of course... wouldn't take 'em on a road trip this way) they don't mind it at all. Except being at the vet's in general :P