I have an old C20 that I have mounted up front. I mounted the speedo sensor on the front fork and the cadence on the bottom tube with a long cable tie. You could splice in another head without corrupting the signal, but I would just mount another harness on the rear and run the wire to the stoker bar in the conventional way-like on a single. This way, you have redundant systems and the old C20's (with their longer rear mount harness) can be found cheaply on ebay. I've had this old Vetta unit for 20 years and it just keeps on ticking!