There can only be one aggressor, and if they are living together, usually one is stressed out and getting nipped at till they die. Healthy male bettas can not live together, however, I have seen the less healthy betta that were too sick to dominate any areas and therefore, there wasn't much nipping or biting, only in super close contact. You can have sibling male juveniles living together, but maturity is inevitable, and no adult betta is known to live with other adult male bettas peacefully. Basically it is their nature and they fight because they need to dominate. Dominance and aggression is what tends to keep bettas healthy and vibrant in colors. You are not likely to have two or more male bettas dominating one open area.