Honestly? I'd have to break it up. Fantasy and reality can be two different things sometimes. My attractions to catfights started in my teenage years, then I went to uni. One night while leaving the uni bar the womens teams of both rugby and football had a falling out which ended up as one girl going for another. In my head it wished it to be...well semi-erotic I guess. The reality was shockingly different, as one of the rugby girls grabbed a football girls hair from behind and smashed a fist into her nose. Girl in question was pretty much conked out by the blow and was in the process of being pulled back into a baying crowd of rugby girls.
Being a human being I had to step in, as it was obvious if the footy girl got pulled in it would be curtains for her. I'd seen bad stuff before but nothing like this.
Whilst i've told girlfriends of my fantasy as yet it's never happened for me, but even if it did, as i've always stated to my partner at the time(if she'd suggested she was so inclined to catfight for me) i'd have to have some rules as unfortunately, in tense situations we can lash out in entirely the wrong ways.
If it was at all possible to discuss it with said 'other' woman and she was also that way inclined then, yes almost certainly i'd let it continue. As such...I'd have to hold a moral high ground and step in. Might be wrong or an idiot but i'd never want to see someone get seriously hurt.