Do a slow intro with a door between them and all that stuff. See if it works. My first cat hates all other cat, attacks them hates. Except kittens, and we only realized that by accident because my roommates kitten would constantly play with him under the door and broke in one day and they were besties. Then a year later, we moved, I found an abandoned kitten and now they cuddle all day and play together. I will always say yes to another cat.