My two girls do this because of hygiene reasons, which I find very unique and amazing. They will have a system where Litter Box A is for peeing and Litter Box B is for pooping, they are very strict about this. However every two weeks they actually switch it up and use Box B for peeing and Box A for pooping. Cats are very intelligent and hygienic creatures. My girls have a regimen of keeping clean and also expect me to keep their boxes clean every single day lol. Your cats may be the same as mine! Just some very clean cats (: quick little edit, I also keep my boxes in separate corners in case you do as well.