I have a 4-litter pans to 2-kitties ratio, two upstairs and two downstairs. Both of the kitties now have to "christen" all of the litter boxes every a.m. and p.m., when I clean them. Tater Sue is a "litter kicker".. Mai Tai is very prissy and neat, you can hardly tell she's been in there, save "the evidence"..
Here is the weirdest thing: They rotate locations every other day, one cat goes upstairs, one downstairs. I can tell because one day there will be scattered litter upstairs, and it will be neat and tidy downstairs.. The next day, it will be a mess downstairs, and it will be neat and tidy upstairs. It is like living with Felix Unger and Oscar Madison from "The Odd Couple".. 
Tater Sue is the new addition, I've had her since New Years 2012. Mai Tai I have had since Christmas 2010. I know that they are still working out some territorial boundaries, as Mai Tai has become more reserved and more of a "bush dweller" post Tater's integration (we're working on boosting her self-confidence), but it is entertaining to see how they are working out the time-sharing schedule with "the facilities"..