do you mean housing or outside? Housing wise, well some people do keep them successfully together but to my mind, chickens need a dry environment whereas ducks are poopy, damp critters

So mine are housed separately and the ducks have access to water and food at all times. They all run together sort of although roughly segregated but thats mainly to protect my garden from the hens-so they free range in the paddock and the ducks have the rest.