Calls are a little more likley to pair up than other domestic ducks, which sometimes makes having more drakes per hen possible.
If she pairs up with one of your drakes it is less likley she and the prefferd drake will allow the other one to mate with her.
Penning her off with her preffered drake is a good option for ensuring you know who is mating with whom. Something to consider there though is, I find that any time ducks have been seperated from each other there is a reintroduction peroid where you may be dealing with them not getting along. They sort of have to re-establish pecking order.
And if she does develop a bond with one drake she will not be happy with the switch.
I have a pair that are bonded and if seperated that call girl screams bloody murder.