Thank you for your reply, stanislas
I, too, have several lookout places. The old male uses them a lot, I hadn't seen the new male use them yet.
The younger male does indeed keep a low profile. Before last night, it remained hidden in the substrate at most times. I think they clashed when the younger male went to eat, since they were right in the food bowl when I separated them...
Maybe I'll try to reunite the group later again... I'll keep you updated