I'm just guessing here, but I would imagine that the back of the neck might be just a little bit uncomfortable, what with sliding the palm of your hand, and especially the fleshy part between the thumb and first finger, back and forth over a set of guitar strings. Having the controls mounted where they dig into your leg while sitting also looks a bit uncomfortable. For the application shown in the video, playing bass and guitar parts through a looper, a double neck instrument would seem to make more sense.