I most definitely would not allow them to share the same nest. You are almost certain to end up with either crushed eggs or detached air cells from them moving them around too much. And I would also make sure that no other hens have access to the nest. I learned this lesson early on when one of my buff orpingtons started pecking at eggs and crushing them when they started rocking a couple of days before hatching and mama was off the nest eating. My hens all tend to want to lay their eggs in the same 2 nesting boxes. So now I have a completely separate coop that I move them to when one of my hens goes broody. And I block off the other coop when I have a broody hen so she can set in peace. Your broody hen should only come off the nest once a day to eat, drink, and poop. I would just make sure that you have their feed and water close by. She more than likely won't want to free range. And they will not lay while they are setting. So they only way you will end up with additional eggs is if others have access to the nest, which I revert back to my previous statement.