My dwarfs have a few squabbles now and again but nothing serious. i found that when i took out one of the wheels they were fighting and Bibi got a nasty wound on her rear, but all is healed and well again now i put the wheel back in.
Make sure you have 2 of everything, 2 wheels, 2 shelters, and not using a food bowl and just placing the food in a pile might help if you think one hamster dominates it more than the other
and yes, building nests is what hamsters do best! its perfectly natural behaviour
hope this kinda helped lol
