Yes. But it can be tricky. The key to keeping males together is to purchase two or more brothers at the same time. As long as they are never separated, they should live together relatively peacefully. There will usually be scuffles and dominance struggles, especially when you clean their cage, but as long as no blood is drawn, there is no need to separate them. If your males do seem to be at war more than peace, try and make the cage bigger, and include a house, wheel and food dish for each male so they don’t feel the need to fight over territories. Another good tip is to clean out their cage in sections – do the toys one week, and the litter the next, so there’s always something that still smells like them.
I have successfully kept pairs, trios and even a few large groups of boys together in appropriate-sized cages and have even separated one member of the group to breed, then reintroduced them successfully. I found that going half a day with the boys and half a day with the girl for a few days both kept him familiar enough in the bachelor cage so as not to be attacked, and managed to get his girlfriend pregnant. However, I would only attempt this with a group of boys that were very well established.
Finally, boys tend to go through a sort of puberty at about 6 months old. Some groups can get much more territorial at this time, and I’ve had moderate injuries sustained during this hormonal period. I find that less frequent cage cleans help as they don’t need to constantly re-establish territory. Also, try to treat both mice equally. If one gets picked up, the other gets picked up. Multiple bowls etc as stated above is also useful. All that said, sometimes even brothers cannot remain together forever, and if boys are going to need to be separated permanently, it’s usually round this time.
Whether your boys will remain together with some minor scuffles, or need to be separated for their health and wellbeing, comes down to a common sense decision. Don’t risk the lives of your mice through stubbornness.