I get asked a lot why cats mess outside the litter box. The simplest thing is if you have more than one cat and you have two litter boxes, don't put them right next to each other.
Temple GrandinJust think about it yourself; you don't want to put the [cat's] litter box down the basement because that's too far, on the other hand you don't want to put where everybody is traipsing in and out the back the door.
Temple Grandin