I wouldn't interfere. Horses fight all the time. It's a natural way of communicating who is the more dominant horse out of the two of them. By separating the two horses, you will just be postponing the fight until you re-introduce them again. Furthermore, horses are social animals, therefore separating them could cause more problems, such as separation anxiety.
I would allow them to work out who is the more dominant horse between the two of them. It allows each of them to learn from one another as well as teaching them to stick up for themselves when they have to fight for dominance when a new horse comes along.
Injury is likely to happen during a fight, but why stop horses from being horses? Let them work it out. The horse knows best.
If you can keep them separate I would. From experience my gelding and another horse would fight and they just couldn't figure it out who was boss and we had to seperate them, but you could see how they go first and if they can't sort it out then keep them separate