This guitar belongs to a jazz guitarist friend of mine. It's a classical guitar/fretless bass hybrid, custom built by electric bass builder Rob Allen. It has the D through high E strings of a classical guitar plus the strings of a regular four-string bass guitar (fretless in this case) all on one neck. It has separate bass and treble bridges with transducers so that the two signals can be sent to separate amps. It is a remarkable instrument and my friend plays it remarkably well, playing the melody with his fingers on the four classical strings while playing bass lines with his thumb.
The hypothetical luthiery question that I have is, "Would it be possible to build an acoustic version of this instrument?" I'm thinking that it would be almost impossible. Maybe it could be done using steel strings for the treble side, but my friend is strictly a nylon string player. It would seem to me that for an acoustic bass you would need a large body with stiff bracing, but nylon strings would sound dead on such a body. And if you were to build lightly enough to make the nylon sound good, the bass strings would pull the thing apart.
One idea that came to mind (and I don't know how conceivable it really is) would be to build a twin-chambered guitar. Run a piece of wood down the middle of the body (similar to the way semi-hollow electrics like ES-335's are built, only not as massive) and thus divide it into a beefed-up chamber for the bass side and a more lightly braced chamber voiced for the nylon strings. Soundholes would have to be placed in an unconventional location but, hey, it would be a pretty unconventional instrument anyway.