Has anything changed recently that could have caused problems -- removal of furniture, addition of new furniture, change in who lives in the home or what rooms are accessible, change in other housepets, change in litter brand, etc? New cats (or etc) visible outdoors that didn't used to be there?
Would it be possible to construct extra "cat space", such as an outdoor run (if they are indoor cats) or a long elevated walkway or shelf or something like that?
Another thing to contemplate, or try, might be Feliway, available as a mist-type spray (in Canada,
PetSmart carries it, I am not sure which stores/chains carry it in the US but I know it is widely available). It is not super cheap, like $30 or so for a leetle small spray bottle, and will not ALWAYS solve problems, but SOMETIMES it does so may be worth trying. The idea is that it is a synthetic version of feline facial pheromones and can make cats feel more at ease or 'belonging' in the place where it was sprayed. You don't spray the cats themselves, you spray a single little spritz in a room they often use or on a particularly-contentious piece of furniture, just one spray once a day. No guarantees; but sometimes it does help cool things down.
Honestly, the two times I've had cats live together for years basically-peacefully, then start serious repeated fighting, in both cases it turned out to be a medical problem (one with the aggressor; in the other case, with the one being constantly attacked). I know it isn't the cheapest thing in the world, but if possible, a vet visit for one or both might be in order.
Best of luck,
Pat
