Welcome to the forum. Here's a link > http://www.freedomconcepts.com/fc_pr...nturer-tandem/
to a company that specializes in the design of custom built bikes for special needs folks. You might want to take a look at what they have done with pedal power and go from there. They have some very innovative ideas about brake levers, pedals, seats, etc.
Good luck and keep us posted on your project.