Cats are both territorial and not fond of change, so a supervised, gradual awareness of another pet is the best method for keeping the peace. Patience is a must, because the introduction phase could take anywhere from a few days to several weeks, or longer in some cases. Stick to the animals' preferred pace, and don't force them to be together. Speak in soft, conversational tones to both animals, and spend quality time with each in their separate spaces, not neglecting the resident pet to give the new one extra attention. Letting each animal see the other for brief periods in a neutral room and gradually increasing the exposure, will assure them that there's room for more than one pet.