Cats are very territorial animals. They love to have their own things and they don’t like to share them with other cats. In the household with two or more kitties using multiple litter boxes is a must. In fact, if your cat does her business elsewhere, outside of the litter box, try to add an extra litter box to eliminate this problem.
Some cats will never use a litter box if it already has the smell of a different cat or if it’s too dirty.
If you have one cat, use 2 litter boxes, for two cats, use 3 litter boxes, if you have three cats, use 4 litters box and if you have four cats use 5 litter boxes.
This simple trick will help prevent any territorial issues among your kitties because each of them will be able to have their own clean litter box, plus one more just as their back up toilet.