Even if the cats share the litter box with no sign of rejection they are not happy and sometimes one of them will do its business somewhere else as a sign of protest. (catfooddispensersreviews.com)
Some cats do not like sharing their litter box with other cats while some don't mind at all. (catsofaustralia.com)
This is even more important if they don't share a litter box as any gap can create spots that are more difficult to clean. (selfpetcare.com)