2.1" tires are pretty fat. We use 26x2" tires. Cannondale 700c single frames of old certainly have very little tire clearance even on their touring frames. Are you sure you can fit a 2.1" tire + fender into a 700c tandem frame? For touring in 3rd world countries, steel frames with 26" wheels are preferred as they are easier to repair. IMHO of course.