I've been in this same struggle at my house. Some of what I do (to stay sane) is:

1. Zero tolerance policy towards hissing/fighting. The agressor gets a time-out in a back room for 10 minutes. If that doesn't seem to do the trick, I try the scruffing routine - grabbing them by the scruff of the neck like a mama cat would and pinning them to the ground - not hard, just firm so they know who's in charge (and its not them).
2. Rewarding good behaviour. If they are in the same room and not being bad, then I consider that to be good and I give them attention and praise.
3. Barriers. There's something about breaking eye contact that helps. If I can see tension building, I put a barrier in between - it can be anything - just so that they can't actually stare at each other any more.
4. Watching for the real instigator. In my house, its little girl Jan. She's the one who picks on Strauss, which upsets him and makes him want to locate Sid to vent his agressions. So the tough thing has been to recognize that she is the one starting a lot of the problems.
5. Feed in separate locations. This seems to help at my house.


Best of luck - I know its not fun when they get in their moods.