What if the external rider's seat and pedal drive were fixed to the steering tube. He could be maybe semi-recumbent with his CoG back a little. Crank ahead of the steering tube, under-seat steering attached to the main frame. They'd need a couple assistants to hold the whole thing still for the external rider to get on and off, and who knows what the steering dynamics would be (likely a nightmare) but his drive would be simpler.
Probably not such a great idea, but it's fun to throw what-ifs around.