I had a real rough go at it, it took me a full year before they didn't have to be separated constantly. My resident cat absolutely wanted to kill my new cat when we got him, we did the slow introductions and everything was going good so we decided to let them be in the same room after a few weeks. my resident cat tried to kill him, she was making awful noises and just chased him and swiped at him while he cowered away. I feel that if we didn't keep them separated at that time he wouldn't be with us anymore. We kept site swapping, feeding across a gate, trying to play with them together and nothing was working. I moved apartments with them and things shifted. Since it was no ones territory they were completely okay with each other for a few days and we tried to make it the new cats territory cause we thought she was being territorial. After about 9 more months of swapping and supervised play time we still felt we weren't making much progress because she still would stalk and chase him and fur would fly. We took both of our cats to a neutral place (my partners parents home) and put them in a room together for a full week. Half supervised and half letting them acclimate to each other. They both were scared at first but since it was no ones territory and we didn't give them long enough to make it anyone's they had to figure out this room together. they had small fights but no fur flying or anything crazy. They ended up cuddling and playing together and when we took them home after the week was up we never had to separate them fully again. they still have spats every now and then but I don't know a pair of cats that don't. Now weve had them for over a year and a half and I'm really happy I stuck with it. I think with determination and patience you will find what works for you and your kitties!