Most times, farmers avoid keeping them together in the coop to prevent fighting.
Ducks and chickens fight for many reasons, and some of these reasons include
Ducks Fight the Chickens to Have the Females to Themselves
Raising chickens and ducks without enough females of either species will cause fights that can lead to death. An excellent breeding system should have a proper male-to-female ratio.
Extreme bright light for long periods causes birds to become hostile to one another. Excessive exposure to bright light can be stressful to the birds, and it can make them irritable.