As with most everything bike, sheldon had a page devouted to tandem brakes, many good ideas
http://sheldonbrown.com/tandem-brakes.html
Front and rear Mafacs actuated by a single brake lever sounds like a recipe for poor braking and control. I like the idea of giving over control of the drag brake to the passanger, captain gets standard seperate F&R levers for the cantilevers.