You can use a single litter box, if both buns decide to use the same corner. It also depends if you are free roam or cage. My buns are free roam and I have 6 litters boxes around the apartment. My litter boxes also carry the hay they eat, so they can munch and use the restroom at the same time. I have found that you can't pick where they are going to pee, but you can put a litter where they have decided to use the restroom. Every corner that they decided was going to be a pee spot, I threw a litter box right in the spot and they ended up using the litter box on their own. A lot of trail and error, but this is what works in my household.