Listen to maryz.
Separate them, or tell them to fight where you can't hear them. I put my 3 outside at Christmastime when they were fighting. I said they could come in when they had finished their argument.
A lot of it is about attracting your attention.
Equally, if they are separated, then they can't be physically violent to each other. (Which I agree is when you don't want to just leave them to it)
I also have a rule about violence, that it is unacceptable. I don't hit them, they don't hit/scratch/punch/kick each other.
But never try to adjudicate their arguments.
If they come to tell you about how they have been mistreated, listen and empathise. "oh, I bet that made you angry!" they can then talk it out, and work out how to solve a problem for themselves.
It's really hard, I know. But most people get over it, eventually, particularly if their parents don't take sides. (But we all know of cases where grown up children are still resentful of the way they are played off against each other by parents.)