My story is the same as those who've failed with divided boxes. A few years back I made eight 6-frame boxes, to be run either as 6-frames or divided into 2x 3-frames. (full-size frames, 9" deep)
I have never once had success with both sides when in the divided format, yet two 5-frame stand-alone nuc boxes side-by-side - no problems at all. Guess I must be doing something wrong, as many people swear by the use of divided boxes.
Still, I ain't gonna give up - this coming year I'll be trying divided boxes with half-sized frames (for queen-mating, as opposed to nuc-raising), but as Jim said - with the boxes positioned on top of queen-right colonies. Let's see if that set-up works ...
LJ