A few things I've learned from 25 years of raising large breed dogs is that you never have more than one male in the house no matter the size. Avoid sitting for the following breeds, bully breeds of any kind, huskies, great Danes, rottweilers, German Shepard, Belgian malinois, pugs.
When dogs are together even just playing, watch them at all times for triggers and warning signs. Such as mounting, stiff posture, staring, toy or space guarding. Don't allow faces to get close together when ground sniffing.
These are some of the things I've learned, and that I do while I'm sitting. But even after all that I've still had to break up two fights in the time that I've been pet sitting.
Fights can happen so fast when you least expect it.
As soon as you see that they are not going to get along. I would put them in different rooms And switch them in and out of the room that your in.