Since they are popular in Holland, maybe that is the place to look. They must sell replacement parts.
You might consider a shaft drive, or even contacting Strida/Ming about a belt drive.
Post to recumbent board for more experienced people.
Seems like a soft tail(s) would be better. I think rear suspension would be good at preventing "3 wheeling".
Did you consider a single front wheel? The pedicab people can't all be wrong.
Since the rear wheels are powered independently, what keeps them synchronized?
Ahh, so why is a Quike better than a tandem?