Dana, the idea of shafts and gears is starting to look like a simplest of solutions after all. The shafts don't need a universal joint at the wing fold joint, they can be bellow the wing joint and simply plug into and unplug from a rubberized receptacle on the other shaft. Simple and easy, and reliable and cheap.
This way one or two engines sit sideways behind the pilot and belts to a shaft running cross ways behind the spar. With two engines overrunning clutches can be used to automatically remove a failed engine from the drive system.