You might be able to get all the rabbits in that room to use the same litter box and have just the one, however this is not always the case. (smallanimalchanel.com)
They all share the same litter box so we have to be very careful about the litter we use. (worldsbestcatlitter.com)
If you know someone who owns the same litter box, you can even bring your cat along and see what happens. (tinpaw.com)