If the presence of a new cat is causing the existing cat to house soil, confine the newcomer. Make every attempt to keep the first tenant's life as stable as possible. Once it has been determined that the cat is in good health, training can begin. The combination of confinement and supervised freedom is the method of choice. The cat starts the program in confinement.
Most cats do well in small rooms.
Since the bathroom is an essential one for humans, the cat is not isolated for extended peri ods of time. In addition to those necessary trips to the bathroom, you should make time for 3 to 4 twenty minute sessions with the cat either playing, grooming, talking or feeding. Put a bed for the cat in the room along with some toys. Remember to place dishes and bedding in the corner of the room farthest from the litter box. Some cats may require a space smaller than a room, with no opportunity to choose the wrong spot in order to learn to use the box.