If their ears are flat, that's usually a sign that it's fighting. The best way to break them up is keep a spray bottle at hand with water. Squirt them both and that should defuse the problem.

Having 8 cats, I keep a bottle in every room. Another thing you can do is put marbles or pennies in a soda can and tape it shut. Everytime they fight, rattle it as hard as you can. They hate the noise.

Good luck.