I'm going through this right now. It takes time. First week, completely separate them in their own rooms. Make sure each has their own food, water, litter and bedding. Start getting them used to the other's scents by rubbing a towel on each and swapping the towels or exchanging cat bedding. Second week introduce one to the other -- one in a carrier for safety for about 10-20 minutes a day under your supervision. Third week let them spend about an hour a day under supervision, no carrier. Fourth week, let 'er rip. I'm just in week one, introducing a new male to a female I've only had for two weeks. It's amazing how used to me and territorial she got in that short time. It's stressful, but I can tell they're starting to get used to the fact that there's another kitty in the house. Ah kids!