I’ve had success putting hatchlings with 2 week olds; anything beyond was too big of a size difference for me. It could be awhile until these eggs hatch, in which yes, it’d be best to put up a divider in the brooder, so they may see each other but not touch. I agree with BelovedBirds in getting...
The eggs look pretty lively to me! Definitely not as far along, though.. How strange. Perhaps she laid new eggs in the middle of incubation? Although, it’s not common for a wild duck.
Also, little Donny is adorable. He certainly looks like a Mallard.