Chicago Pet Store
When in a Chicago pet store last week I got a flyer that talked about how to help new cats become comfortable with each other. It said that cats that have been exposed to other cats while growing up might adjust more readily to a new housemate. The best way to avoid conflict between cats is to carefully prepare for the first encounter. The resident cat may adjust to the newly introduced cat without confrontation if they are first required to share your home but at different times of day, and are not immediately introduced.
If the cats are consistently aggressive toward each other, it may be necessary to sedate one or both cats. Psychoactive medication should be prescribed by your veterinarian and used only if necessary and for a short time. Both cats are likely to experience some anxiety during the transition period, though one cat will likely react more strongly than the other. Unless there is a medical reason not to use medication or one cat seems completely unbothered by the other, both cats are likely to benefit from medication. With time, most cats learn to accept others in the household. Should your cats be exceptions, however, three options remain:
Keep one cat confined for part of the day, while the other roams freely.
Keep one cat permanently confined to one part of your home, while the other is kept exclusively in the other. You can always try another introduction later.
Find another home for the new cat you had hoped to adopt.