How close have they gotten to each other? Have you tried putting them in adjacent rooms with a shut door so they could get used to the smell of each other (from the air flow under the door) without having to duke it out? Have you left things with the other cats' smell in the room with them? Hmmmm - maybe some chemical intervention in the form of Rescue Remedy, or maybe catnip, if they're the kind of cat that reacts by getting really mellow afterwards, not if either is the "wild frenzy" reaction to catnip type ... My Dad used to bring catnip mice with him over to Lady's Human's place (before they moved 5 hours away) and so my brother would come home to three "stoned and mellow jello" cats lying about.