Ok, thankyou! I thought I would be fine letting nature take its course, as you say, they've been fine sharing the nest this whole time! But I'd just read recently in a duck book that you shouldn't put two broods in together, but maybe that was based on the fact that they had initially nested...
I know this was a couple of years ago now, but how did it all work out? I have the same issue, two ducks one nest, babies due in a few days, don't want to interfere but don't want them to kill any babies either!