I asked this same question elsewhere lately. I was told they can be together for a couple days then the ducklings need a separate brooder. They are messy with water and make everything wet which is unhealthy for the chicks.
One super tip I got was to place the ducks water and feed on wire (perhaps a small box with a wire top) with a tray under the water to catch splashes. They also suggested using outdoor carpeting instead of wood chips. If you had two pieces they could be alternated to allow one to dry. I plan to use both these ideas in my brooder.