I get the idea they are bored with winter and probably each other too.
Photos attached...
How deep is the litter on the floor?
If it's shallow, maybe add some more.
If it's deep, maybe turn it over in a few places to make it more interesting.
The reason I suggest this: it might cause them to spend more time scratching, so they have less time & energy for picking at each other, but without giving them a lot of extra calories by adding treats. It may or may not help, but probably does no harm either.