It cracks me up when they do that! I have a coop with 6 lovely nest boxes, and they'll stand in line or pile up to use the same one. I've actually even seen a hen lay an egg while sitting on top of another hen!
You should really blow their little minds, and put the pink lid on the blue box, and the blue lid on the pink box. Wonder if it would change things?? Maybe, they are laying ALL pullets in the pink box, and are reserving the blue box for the roo eggs??