They are fighting for dominance most likely.. Hens maybe but i've had roosters do fine with 2-3 hens.. So go figure.
You can either sell them, seperate them, or you can let them fight it out.. I recommend the first two but i guess if your into the whole "let nature take its course" then you can go with the third one. It's how it'd be settled without human contact.
ETA: The best options are to sell or seperate though, because leaving them together does not mean they will not fight again.. Anytime something happens "they spot the other rooster breeding a hen, feed, they just get that "frisky feeling" ", they are males.. And we get pretty jealous over stuff like that. Or at least a lot do.