They won't kill one another ! This is surely what Lankeela is describing; rough housing by youngsters. Perfectly normal.
My lot of little ones fight like demons sometimes, and have done regardless of age, or age differences, over everything and nothing; very alarming if you're not used to seeing 5 piling in, sometimes onto the one who's losing (they have no idea of fair play), with screaming and fur flying, and each having parallel fights with any who's in the way; but the whole thing ends quite quickly and they're all friends again within minutes. Blood is drawn regularly in these encounters, though very little harm is done. Intervening is rash; in the excitement, they'll bite the human when trying to bite each other!
You've got the same thing but on a much smaller scale, with two. They'll settle down. There may be conflicts to come, but they won't amount to much or be frequent.