I really don't recommend it for most people. If the birds are all fully grown, you have a large enough area, multiple feed and nesting areas, and your duck to drake ration is good- then you should be fine. For most people though it is safer to keep them separated. It may be rare to have problems, but problems can equal dead ducks. One time I recommend never mixing them is when they are ducklings after the age of about 2 weeks old. As adults though, with adequate precautions, you can sometimes do fine with mixing them.