A duckling does not need to imprint on a human in order to become a pet duck. It is highly unlikely that any person is going to have the time to devote to a single duckling for the next 10-15 years. Yes, that is how long they will live. Personally, I think that raising single ducklngs is basically cruel. Nobody knows what the future will bring and nobody can say for sure that in 5 years or even 1 year they will be in the exact same living situation. Ducks need companionship and they do best with other ducks as that companion since the ducks will be together all the time.
In order to get a people friendly duckling, time needs to be spent with it. Talking quietly, holding and handling. Breed often plays a large role in whether or not a duck is going to be seen as friendly by a person. Some breeds are just naturally more nervous than others. Just because there are 2 ducks does not mean that they will dislike people and not bond with their owners. It just gives them a companion to play with, sleep with and act like a duck with.