Mine fight all the time but is usually it seems like it's 5pm, time to test each other, it's 5.10pm, time to test each other, etc. It's serious but almost like a ritual as well on placement of bites. I've watched a few times where an 'attack' seemed to get broken off by one if the other one was distracted. Or maybe I'm just reading too much into it.
Anyways they do hang out together but other times need plenty of space to escape. Andy would know more here. In my tank if one or a pair decide this is their spot for a few days then it seems like the territory is about a square foot.
I have 5 in 150gal and they are mostly pretty good. Don't bother other (large) fish (unless the Congo tetras are trying to drill through the glass) and put up with getting run over by every fish at feeding time (plus the loaches any other time of day).