Have they been neutered? Have they actually had a physical fight?
Cats do an awful lot of posturing when they first meet? It often looks and sounds a lot worse than it is.
What I would do in this situation is have them both in the same room. Get a large cage, make it comfy with a blanket and a cushion. Let them take turns in the cage for a while, just so that they can get agood look at one another. They need to be able to check out the scent of each other also.
If they are taking turns in the cage, their scents will mingle. One of the cats will have to be the dominant one. Sooner or later they will have to come face to face. Usually one will back down. If you introduce them whilst one of them has got the protection of the cage, that will take the stress out of it for you. They will not be able to tear each other to pieces. In my experience, cats will learn to tolerate each other, even if they dont like each other.