Cool hive! I have gaps, and the bees can go in the back, but they don't do much back there anyway. It shouldn't be a problem.
Good point, I was assuming you where planning to put a colony on each side. With the screen it is not a problem. If this is a 1 colony hive than the screen is not needed. For a one colony hive, giving them access to the empty space behind the divider will allow them to keep spiders, ants, other insect from taking up residence back there.