Real fighting is heavy no holds barred, cats locked together, in a roll, hard to seperate.
Mock fighting is when they are posturing, circling each other, ears laid back, tails switching, quick swipe of hit and then apart. This is their survival hunting instincts kicking in and how they interact with each other. If you have two cats locked in battle, don't get between them yourself, throw water on them, turn the vacuum on startle them somehow, but don't get in between such a fierce confrontation. Anything from a long sheet of tin foil flapped at them to cans full of rocks to startle them should get them apart.