Thanks again Doug. Reading you explanation and studying the pictures again I now understand how it works. I see now that a second set of pedals would be equivalent to the rider both changing gear and changing his stroke at the same time so that they would not provide any real benefit.
One question however. Are the pedals arms connected by some sort of "see-saw" linkage so that they always move equal and opposite? If so, would not this enable the rider to pull as well as push? If the pedals were not interconnected, he could use different length of stroke on each pedal or even give one leg a rest altogether. He could also use both legs together, a bit like a swimmer doing a "dolphin kick". By pushing against his shoulders rather than his lower back, he could use his back muscles as well as thigh muscles. Possibly opening up new possibilities of pain and injury not previously available to him!