What To Do?

I've had that happen.

I didn't do anything about it, although I should have.

Next time I will split them into two spots with a divider between them to prevent competition.

Every time one hen would leave the nest, the other would steal eggs by slowly rolling them underneath her side.  Sometimes that side would be nearly empty when the other one returned.  She would steal eggs back at the next opportunity.  It was a constant back and forth.

More than half the eggs still hatched.  Ducklings were a bit confused and didn't follow either hen very well.  They would decide to leave a hen and run to the other one who was yards away.

I think the competition lowered the number hatched and stressed the ducklings.