The bees do restructure the resources as the two nests merge. No, I would not say they rob any box. It is more a matter of their reorganizing of the contents to most efficiently support the combined brood nest.
The point and the perspective to take is, after a few days the setup is no longer two colonies. It is one colony, one nest, managed as one, resourced as one. It just so happens to have a queen at each end of it.