Ok I had a similar situation with my "butt head" cat and the new littel girl I got to keep him company. They hated each other or more accuratly he hated her. She spent all of the time in my lap or in hiding to stay away from him. The way I solved this was by accident. They both needed to go to the vet and so my daughter and I took them. I had one and she had the other. Well the whole vet thing throws them for a loop anyway and this time was no diffrent. I had to pay at the end of the visit. We had been keeping them apart the whole time. I gave the little girl to my daughter. I asked her if she thought she could handle them both for a few mins. She thought she could. She put the little girl on top of the bad boy. He was so distracted by the strange surroundings he didn't even seem to notice she was there. When we got home and put them back in their own environment there was no more problem. No hissing or fighting or running. They have gotten along fine ever since. So I say take them to a new surrounding and introduce them all over again. It is worth a try.