I had 2 broody Runners nesting next to each other, swapping eggs the whole time. When hatch time came, they stayed together. I tried to seperate them on 3 occasions but they went nuts - became very aggressive to other ducks, paced at the fence seperating them, etc. I relented & let them raise the ducklings together. They still herd their 6 week olds around as a team. Ducks can co-jointly raise ducklings but it is the exception rather than the rule. There is still the risk of being trampled by 2 Moms but, as I saw, a single Mom can stomp on a duckling & kill it just as well. You just have to monitor their behaviour really close when the eggs start to hatch. I try to let nature take it's course and not intervene but sometimes I have this uncontrollable urge to help smaller, weaker hatchlings. It's foolish, I know.