Betta fish have been bred for hundreds of years to do two things. Look pretty, and fight to the death. The males are especially aggressive. In fact, most prefer to be alone. In my experience, males that are kept in divided tanks tend to have a slower healing process than those who are kept alone. Having another nearby betta that is assumed to be a threat to it's territory is stressful, and therefore not recommended to anyone but the most experienced betta keepers. That goes for betta sororities too. If you want anything close to that to succeed, you need to study betta behavior, and research, research, research! Rarely does keeping any bettas, (male or female) together yield positive results in the end if the owner has next to no clue what they're doing when it comes to knowing their fish's anatomy, behavior, personality, etc. I would NEVER recommend to keep more than one male in the same tank without a divider. Never. Too many risks involved, and the fish will be unhappy, and therefore live a shorter, less fulfilling life in the end. The idea of experimenting with living creatures (especially pets!) like that just makes me sick to my stomach.
Winston~ Damian~ Twitch ~ Evans