There's been some very pretty looking concepts done using the joined wing style but a quick google search didn't really find them. I also found that the style is reffered to as a box wing like you used but I always thought of a box wing as more like a biplane with full tip panels much like the Santo Dumont canard biplane.

Anyhow, I did find some sites with the term "joined wing" so there's info out there. In the meantime there's no reason that you can't get your own design tests running. Make some models out of sheet balsa as simple test gliders to experiment with stability and CG placement and have fun with it.

A lot of a normal airplane's drag comes from the energy taken to form the vortices at the wingtips. The joined wing idea, if done well, may be one answer to limiting this loss. Some variations would certainly be fun to play with.