In the question What should we look for in a kitty companion for our rabbit? we learned that cats and rabbits can make good friends for each other, but it was stated the litter boxes could be a problem. What do I need know or do concerning litter boxes when a rabbit and cat share the same home.
You likely do not want a rabbit to share a littler pan with a cat (or dog) due to possible parasite transfer.
Cat (and dog) feces may contain worm eggs. Contact with this is the primary way a house rabbit can contract worms.
Another issue you may run into with sharing a litter box is odor, and while you may be tempted to use cat litter for it's odor fighting properties, this can be harmful to a rabbit. Rabbits have sensitive airway passages and the dust form clay litter can cause a lot of problems.
Instead, ensure that the cat box is in a place only accessible to the cats and not the rabbits.
In contrast, indoor cats with low exposure to outside animals (like mice), are much less likely to be worm egg carriers.
If you are going to try to share a litter pan, then pick the litter that's best for the rabbit not the cat.
Use hardwood pellets
I have personally found that hardwood pellets make the best litter for a rabbit as it is edible and does the best job at moping up their pee (which is usually what makes the box stinky). However, while ideal for rabbits, it is likely not going to do a great job at masking the smell of cat feces.
If the smell is intolerable, then you should not combine litter pans.
ref: Worming your Rabbit