We had a single cat who had to be PTS last month, her litter tray was sited by the back door, no problem.
Thing is, I would love to adopt a pair of cats but cannot begin to imagine where we would put a second (or third?) litter box. We are in a typical 1930s three bedroom semi, including a seperate WC to the rest of the bathroom - no space in the bathroom (hardly space to move!), none on the landing, none in the downstairs entryway/hallway.