I am interested to know that as well - ducks out of the coop.
My coop was nice and perfect size for my chickens.... until I put the ducks out side.
The duck house is actually nicer than the chicken coop, but I guess the "water chickens" want to stay with the other chickens. The only way I can keep the ducks out of the chicken coop is to close the door. However, when the door is closed the ducks sleep right outside the door, they wont go to their house.
Now I let them all stay together. It requires a little more frequent cleaning but all the chickens, water and regular, seem to be happy.
Now the chickens use the duck house as their day-house for nappy-time.