Don't worry about it - cats fight with each other all the time - its perfectly naturally - its best not to get inbetween it - unless one of the cats is getting badly hurt.
I remember when I went to the rescue cattery to get Confucious (nearly 10 years ago

) and while I was filling in the paper work for her - this man came in with a cat in a carrier saying he did not want it any-more. He had got the cat a couple of weeks ago along with another one and he didn't like it when the cats fought with each other. The assistant explained that it was perfectly natural behaviour (the two cats had always been with each other and were great with each other!) and this is what they called play fighting. But the man said he did not want his children seeing cats fighting because then they would think it was alright to fight

You have to laugh, otherwise you would cry. Those poor cats were then split up because of that man!
Have you got any books on multi cat households? If not I really recommend on reading up about it.
