One thing that will help when two hens set the same nest is to make sure they have lots of nesting material in the nest box so they are cramped together and less likely to chill eggs.
One word of caution about letting two hens set the same nest. If you can get them to set in separate nests your results will be much better. I have noticed when two hens set in the same nest some of the eggs get ruined because when they divide up the eggs some will not get covered between them...