I have two bonded girls who went broody a week apart last year so I let them sit and hatch. I had 2 nests, but they would still sometimes squabble over one. I'd be wary of only 1 egg; I'd recommend 2 nests with 2 eggs minimum. Also when Ethel's goslings hatched Fredericka abandoned her nest and tried to steal all the babies. But I don't remember it being WW3 like my 2 mamma ducks fighting over 1 duckling.
Ethel would chase everyone out the coop and I had to fence off overprotective mamma goose and goslings -- especially at night. Free ranging they were all one gaggle. Bedtime was goslings and Ethel, close the fence, ducks into their side, Aunt Fred next to the babies, fence, then Ricky and Lucy. Ricky wasn't very fatherly beyond generic gander duty.
You'll also want to protect your ducks. I have duck sized escape routes in the coop where they can run from the geese. I also put up some wide edging/fencing that the ducks could run through while free ranging that is too narrow for a charging goose. Youll need separate food, water, and bathing if you don't already.
Don't forget baby grows up and will need a home (or butchering.)
Edit: Food didn't change -- Purina ducks pellets. I saved the bottom of the bag dust for the babies.