The short answer, with the condition that matters
A mixed flock of ducks and chickens can share one feed successfully, and plenty of keepers run the whole yard on a single all-flock or waterfowl ration. The arrangement fails when the shared feed is chosen for the chickens alone and the ducks are expected to make do, because the nutritional gap between the species is real even though both birds look similar at the feeder.
Niacin: the duck requirement chicken feed misses
Ducks need substantially more niacin than chickens, and niacin is the nutrient that builds sound legs and steady growth in young ducks especially. Ducklings raised on plain chick ration can develop weak, bowed or turned legs as they grow, while the same feed raises chicks perfectly well. A waterfowl starter solves this directly; where only chicken feed is available, keepers commonly add a niacin source for the ducks. Adult ducks on range with greens are less vulnerable than fast-growing ducklings in a pen, but the requirement never disappears.
The layer-feed and calcium question
Layer rations carry extra calcium for hens producing shells, which suits laying ducks too. It does not suit drakes, young birds or non-laying ducks over the long term, because a bird not building shells has nowhere to put that calcium. Flocks with drakes often do best on an all-flock ration for everyone, with crushed oyster shell offered separately so that only the layers help themselves to extra calcium as they need it.
Water is part of the feed
Ducks eat by scooping a mouthful of feed and washing it down, and a duck trying to swallow dry pellets without water can choke. Keep a waterer deep enough for head-dunking right beside the feeder — accepting the splashing and mess that comes with it — and site the feeding station on gravel or ground you can hose down, because a mixed flock's dining area gets wet and muddy fast.
Running one feeder without short-changing anyone
Set out enough feeder space that chickens cannot guard the whole ration from slower ducks, and put feed down at predictable times so both species learn the routine. Watch the ducks at feeding for a week whenever you change rations: bright, active ducks that come running are telling you the system works. Because two species on one yard is exactly the kind of routine that drifts between helpers, this is where a written feeding plan earns its keep.
When the feed is not the problem it seems
Weak or wobbling legs in a growing duckling, a duck falling behind the flock, or birds going off feed despite a sound ration deserve a veterinarian's eye rather than endless feed changes — nutrition explains a lot, but not everything, and leg problems caught early fare better.
KinsLe's Care Teams fit a mixed flock neatly: the day's feeding — which ration, which supplements, water refreshed — sits as an assigned duty for whoever is on yard rounds, and the finished job is marked with a timestamp so nothing is doubled or missed.