I had a cat, Naomi who was 4 years old when we brought home a tuxedo kitten named Elvis. They never got along. She hissed at him her last day on earth, when she was over 18 years old. They never seemed to hurt each other and always stayed in the same room of a 4 room apartment. They had a choice and seemed always to be in the same room. So that was just the way they were and we got used to it. The next time, I will get two cats who get along from the start. I think that Elvis chasing her around kept her young. But I don't know how you can convince them to like each other.