We also use Tidy Cat for multiple cats, and it works well for us. When we were a 2 cat household, we started out with 2 boxes, but realized our 2 cats were sharing the same box after awhile. Then, we went with one large box, which worked well. Now that we have 3 cats, we are back to using 2 large boxes. Maybe you could get a second, small, inexpensive box for when the new arrival comes home, and when the 2 have adapted to one another and are using the 2 boxes interchangeably, move to using one large box.