Gordon Stowe has problems because the two brood boxes are effectively welded
together during the summer months.  Where one comb is above and in line with
another the bees very sensibly join them together.  The solution is to have
the upper box at right angles to the lower (or vice versa).  The bees then
respect the bee space between the combs in the two boxes and do not join them
with brace or burr comb.
Of course, there are eccentric people who choose to use oblong rather than
square boxes.  They can't do this and have only themselves to blame.
Chris Slade