Have both of these boys been neutered? It might help ...
You are not going to like what I write but here goes....this came off of another dog forum and I have never tried it but it does make sense when two dogs are fightening and you can't break them up.
Stick your thumb up their butt! That will get their attention...and I am being serious..
I will tell you I had two big boys (both neutered) that just did not get along and we could go months without a problem and something would happen (I never could figure out the button but in seconds they would be trying to kill each other and then everyone else jumped in...it was awful...so I understand why it frightens you. Sadly one of them passed with cancer so I never did figure it out nor did I every have to use my thumb.
I will say the faster you jump into the fight the quicker it will end without a lot of damage to the dogs.