Introduce them carefully, and they'll have a better chance of getting along. Bring the new cat into one room with a door, and leave her there at first, letting them get used to the smell of each other under the door. Then make sure you still pay attention to him just as much, even maybe more at first! And supervise them carefully at first. Don't worry about spitting and hissing - it's just part of settling who is who in their new hierarchy.

There will be a period of adjustment, but I bet they will get along eventually!