Definitely Two Litter Boxes ,at least at first , as they will be territorial , for a while , even if the New Arrival is a Girl Cat! At The Found Cats I have 7 boxes , that are clened twice daily , as Moose The Magnificent and JJJ3 kick everything out , of thier Box , if it has been used. And Best Of Luck with the New Arrival , you are making space for another Cat!