Hi Alan, I find one brood box to be “just right”. The key is to make sure that all the frames in the brood are A1 for producing workers. With that strategy in place, the hive will be overflowing with bees in no time, given the right conditions.
My experience with 2 brood boxes was: when there was a dearth, the bees concentrated on producing brood in the top box, leaving the bottom box with no brood but lots of pollen. These frames not getting guarded by workers as much as they guard the brood, leaves those frames in the bottom box vulnerable to SHB damage, if one happens to be in an area where SHB is a problem.
It does make swarm prevention harder with 2 brood boxes. In the right conditions, the bees certainly build up strong with 2 brood boxes, however, if the colony swarms, & you don’t catch it, you lose a LOT of bees.
Added to the list of cons is the extra expense & time involved in preparing the second super. I believe that the extra super & frames is better served with a second colony.
I’ve been saying this for a while now that I reckon two queens are better than one.