I've seen on Micheal Palmer's videos where he makes a double nuc box with an eight frame deep and a solid divider down the middle. Additional frames are added to the top with single four frame boxes. The advantage is in overwintering as the clusters from both nucs cluster in the center against the divider to make efficient use of heat from both clusters.
I've also read where two brood chambers (with two queens) can be stacked on each other with a queen excluder ( process over-simplified) and the two colonies act as one with two queens.
Has anyone tried a double nuc with a queen excluder for a divider? I'd think one would have better heat transfer and the advantage of the two to pool resources and act as one.