Hi there Peachie, I dont know of many other people out here who have ducks sitting at the moment. I am in Melbourne- and its freezing cold and very wet this winter. My ducks started laying a few months ago so I have been incubating Indian Runners for the last few months instead of Pekins. They have probably jumped off the nests for a few minutes when you didnt see them- they will come off for a few minutes- even up to half an hour each day and the eggs will be fine. I dont keep muscovies- so maybe someone else can answer the question about how common it is for them to nest so close together- but they have a reputation of being very good mothers- so will probably sit until they hatch out the babies. Just watch to make sure that neither of them is stealing eggs from the other- or that they end up with eggs in between them that neither is actually sitting on. Also when they are due- I suggest you keep a close eye on the mothers- as they may try to take babies from each other- or even hurt each others babies. It isnt very common- but just something to watch out for.
Best of luck to you and your two ducks for a good hatch in about 32 days!