This is with 90% of males animals in the world, mice are no different. Its all about dominance, and territory. I would separate them and put them each with a female, before one kills the other. I'm not a tree huger but you are doing these mice more harm by letting them fight till one kills the other, at least one could have been food.
These two males will continue to fight, separate them and if you cant use one as a feeder.
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