Going along with Sandie's idea of limited contact, sometimes any barrier between cats is enough to make them more comfortable. When I took my cat over to a friend's to see if her cat and my cat (both neutered males) would get along, the cats put barriers between themselves (a chair, the door of a pet carrier). My cat seemed less interested in playing but my friend's cat (who I think is a bit younger) was very interested. When the cats were on opposite sides of a barrier, the younger cat would reach out to touch the older cat, and the older cat would play as well. When they were out in the open, the older cat would hiss, and the younger cat would swat.
Perhaps you could place a baby gate or some other barrier between the cats so that they could still see each other and determine if that helps them to be more relaxed around each other.
Also, when I lived in a more rural area ages ago, I had a spayed indoor/outdoor cat (she was left behind by the previous home owners because they said she was too wild to catch). The previous owners, who apparently didn't fix any of their cats, asked about a year later if I'd take my cat's nephew - a kitten at the time. The female cat would hiss and swat at him. He was fixed not long after I got him. He grew to be bigger than his aunt and then he bullied her. Eventually, they learned to tolerate each other. They would go off into the woods together and often return with some sort of prey (I used to call them Killer Kitties).
So, give your cats some time. Hopefully they'll start to get along better.