Ahhh, my cats always used to play when they were younger, they would get either side of the floor length curtain and paw scrabble, then one would 'jump' the other througth the curtain and that would lead to chase fights round and under the curtain! I would start to realise if one got a bit fed up though as the tail would start to wag madly and then the hiss together with the slap of a paw would make the other back off for a while!
As nosha said, it is great that they get on together and like to play, although if the hisses are increasing it may be that one of them suffered too rough play and is now a bit wary? I am sure that things will settle down again and they will sort out any differences!
Cheers Sue