chickens dig, scratch and poo alot. rabbits dont. not something we would ever do as they both have different needs tbh. one of our bunnies - aka the worlds most miserabilist rabbit - would probably kill a chicken at 20 paces with one of her cold stares. the only rabbit i know who bites and kicks.
I think my buns would be pretty miffed to be sharing with hens tbh. Rabbits are very clean animals, they should be litter tray trained and won't soil their sleeping areas. Chickens perch to sleep and so poo everywhere. Chickens can also eat a wide range of scraps whereas rabbits are prone to gut status and would get ill on a lot of things that are fine for chickens.
Just a thought - we have a pair of rabbits in a hutch with a big run. Could they live with a couple of chickens? What about sharing a run, but with a seperate house for the chickens, where they could perch and be out of the way of the rabbits?