If your ducks and chickens are over 18 weeks, you can rest assured knowing that they can eat the same feed. I recommend giving them pellets or mash. The only major difference for ducks is the lack of niacin in the feed. Here's a link to an example of the feed you could buy for both chickens and ducks.
https://henuporganics.com/products/layer
How big is your entire flock mixed together? How many chickens and ducks do you have in total? If some of the chickens and ducks are still under 18 weeks, simply purchase a small kennel that can house your hens and ducks. Then, buy maybe three nesting boxes. Example:
https://www.amazon.com/Miller-Manuf...eywords=poultry+nesting&qid=1650373509&sr=8-5
Just buy a kennel, nesting boxes, straw, and whatever makes it feel like a chicken coop till they get over 18 weeks. But make sure to keep the kennel close to the main coop. That way they will still get used to each other while being separate because of their food. Hope this helps! And yes, its important yet annoying that you must put them on different feed because of ages, but its important for getting their protein. And again, your chickens and ducks can surely eat the same feed!
