I know that some of you say one. But when Alley came to live here, she started using the same BIG litterbox as Persi. Simple solution. Use twice as much litter and scoop twice as often. When we got little Shelby she had her own litterbox but Persi started training her to go in the big litter box and she never uses hers. Right now three cats are using the same litterbox. So, 2 1/2 times the litter and 2 1/2 times the scooping, right? I mean as long as everybody is happy should there be a reason to force them to use separate litterboxes?