It's very possible that the dominant drake would claim both hens. Poultry society can frequently appear cruel, but 'it is what it is'. Your concern is totally understandable.
Sadly enough what you are seeing is pretty normal duck behavior. We are rapidly approaching the breeding season, and the dominant drake does not want the subordinate near his mate. It's natures way of assuring that HIS genes get to carry on. As long as the subordinate is not being physically...