This is indeed strange. We have never heard about this type of situation. There are several possibilities. One, a human is moving the eggs, the swans are juveniles and have no idea what they are doing and are going from one nest to the other. Or, there are two females. Yes, they will mate, bond and lay eggs, but the eggs will be infertile. This seems to be the most plausible explanation. You may have two female swans who have each built a nest and are sitting on their respective eggs or alternating nests to be with the other swan. The Regal Swan