I agree with Greg the Lionhead I’ve bonded six rabbits and I have five litters. I have two rabbits that will pee constantly on the floor drives me nuts the others they seem to be OK my one girl she stands outside of her litter and eats the hay. Maybe yours is doing the same I have a large high sided cat litter. I’ve seen all six of them in one. I don’t know how they eat the hay like that but and then I’ve only seen one. I’m just wondering though also, are you sure that your rabbits are bonded and not just getting along?  not trying to offend just  a question 😇 i’m going to cut my litter boxes down make it easier for them to get in cause I really don’t think they like that part having to jump in.  I wish you good luck and really hope you find a solution to keep them together with you.