Socially affiliated cats, which are two or more cats that are familiar to each other,share a territory, and exhibit behaviors such as grooming, playing, or resting together, may be more willing to share litter boxes. Because more than one social group may occur in a home, providing adequate resources for each group is important to decrease the chance of adverse behaviors.
For more information from the AAFP and ICC on cats and house-soiling, see their brochure here, sponsored by Ceva.