In a multicat household, more than one box will be needed. This is not just because one box gets dirty too quickly (although it does), but also because some cats object to sharing and one cat may be too intimidated to pass another cat in order to get to the cat’s area. The litter box rule: have the same number of boxes as you have cats.
Multicat households can create litter box placement problems. If there are any territorial disputes going on or if your cats don’t especially care for each other, the boxes should be place far enough apart so that if one is being guarded, there is easy access to another. It’s worth taking the time to pay attention to the areas of your home where each cat seems to spend the most time. You may be able to avoid future problems by placing boxes in rooms within each cat’s general preferred locations. Boxes spread throughout the house are better than ones grouped together in one location. It may not initially seem very convenient as far as you’re concerned, but trust me, there is tremendous convenience in not having to deal with location-aversion litter box problems.
For more advice on ways raising a well-adjusted and -behaved cat, read Think Like a Cat from www.tuftscatnip.com.