Though many of the care sheets don't mention it most of us here with two or more FBT's can tell you that they seem to have a social pecking order. One of the males will dominate the other males and females. And this does include jumping on the backs of others and holding them tightly in an embrace similar to mating at which time you might hear the submissive toad let out a sort of warbling noise. I take that as it saying "Uncle".
At times this does include holding the other underwater for lengthy periods of time. Sometimes it seems to be very violent, but they are not going to do any damage to each other. And FBT's can handle an extended period under water with no harm. They are amphibians and they can absorb oxygen from the water through their skin. Or so I've read.
If you browse the forum you'll see many a newb asking about this behavior. It's just something you have to get used to.
As for them hiding all day. Many do. My dominate male is the only one that stays out and about most of the "day". The others spend more time hiding. Though some do come out for a little. Many times just to have the dominate one jump on them.
As for lack of activity................... well they don't do a lot. They tend to just lounge around thinking about their next meal. It's not like they hunt all day or have stuff to do.