My husband and I have a cat named Cleo and one day decided to get another cat, whom we called Otis.
Otis was twice the size of Cleo, but immediately took to everyone and everything he saw. Cleo, on the other hand had nothing but contempt for the intruding cat and followed him constantly, hissing and growling.
After a month of this kind of activity we had had enough. We were talking to a friend about the situation and he facetiously suggested shutting both cats in the bathroom overnight. We were just crazy enough to try it and miraculously it was a success. The two of them were best of friends after that.
Actually, Otis ate too many looney muffins and we had to give him to someone else, but that is another story.
-case