I think that they are either both competing for the nest box or the hen wood duck is dumping an egg on the merganser clutch. Woodies are more likely to dump nest than mergansers but both will do it.
Dave its called nest or brood parasitism, happens in a number of bird species, probably most famously in brown headed cowbirds or cuckoos. In waterfowl in happens in a variety of ways from same species to different species. Redheads parasitize canvasbacks and other species, while wood ducks, hooded mergansers, goldeneye will dump eggs in nests of hens of the same species or maybe a different species. There are lots of theories on why this happens but I'll save that for another time.