Watching two fish trying to assert themselves over each other is always alarming to watch but it is quite natural. It is frightening and fascinating to watch but two fish like this will just naturally duke it out til one backs down...it can take hours or days before peace entails. Sometimes they won't get on and there will always be aggressive sniping.
As horrible as it can be to watch, sometimes you have to just sit back and let nature take its course. They need to sort out their grievances and hierachy in their own way.