I wish I took more note of your post, Adam. I've just discovered the lack-of-bee-space problem when using two MB brood boxes. The space between the frames is about 5mm. Can a bee squeeze through that?
I'll buy another roof and floor so that I can use the two brood boxes as single-brood hives, and then that's it - no more MB nationals.
Kitta
I've just discovered
Dave Cushman's page on bee spaces. It seems bees can use a 5mm gap and because the box is polystyrene, the sides will not shrink, so that 5mm shouldn't reduce - but still, it's perhaps a bit tight.
PPS: I found a pair of callipers. The space between the frames is 4mm so, according to DC, only usable by the smallest of bees.