##If you are going to try to share a litter pan, then pick the litter that's best for the rabbit not the cat.

The 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.   

###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. Instead, **ensure that the cat box is in a place only acceptable to the cats and not the rabbits**.