Things seem to be settling down between Truffle the Demon Spawn and Licorice, Basement Cat's Minion. While it might be too much of a stretch to say they are making friends, they can sit in the same room and ignore each other for an hour or more at a time. Then one or the other will make a wrong move, resulting in yowls, a chase, and Truffle either going outside or to the bedrooms. There is a door that closes off the hall leading to our bedrooms and baths, and that end of the house is now Truffle's at night or during the day if he needs a nap or just a break from Licorice.
Truffle has had five years to adjust himself to our schedule, so while he does take a long nap during the day and does some roaming at night, he is not as nocturnal as Licorice, who sleeps most of the day and plays in the living room at night. We have tried letting Licorice outside, and he went out eagerly the first time, at which point he realized that hot and steamy does not compare with air-conditioned and filled with cushions. So at least until fall, he is an indoor cat. Truffle is an indoor, outdoor, indoor, outdoor, indoor, outdoor cat. Whenever one of us goes in or out the door during the day, Truffle goes through it, too, frequently in the opposite direction. He's usually in by 10 PM.
Licorice still has an unsettled stomach and interesting grooming habits. Our vet is testing for parasites but hasn't found anything yet. She thinks the stress of living with a large group of cats could also have been the cause. I'm warming up to him. He likes being petted and is friendly to humans, just not to his feline housemate. I keep reminding Licorice that Truffle was here first, like that would mean anything to him.
I hope in a few more weeks I can give both of them the run of the house, at least during the daytime, without having to break up fights. I'm encouraged by how things are going so far.