Boys are not as accepting as girls when it comes to sharing a space. I had my boys separated for two weeks due to a medical procedure, and had to reintroduce them back into the cage together. I broke up a fight that I didn't like (they were rolling around and squeaking) and spent a day monitoring them before I let them stay together. I took out a ramp that they were fighting over, and now they are keeping their distance when they don't want to be bothered. You can keep trying/backtrack a step and try again. Sometimes it's just a matter of dominance. Some owners of male rats get them neutered or get the most aggressive boy neutered to tame his behavior.