Originally Posted by
conspiratemus
2) The stoker might have been given independent control of one rear brake to allow her to actively oppose the captain's daredeviltry on steep descents. But communication is a better way to resolve these conflicts. Braking that surprises (or pisses off) the captain can be dangerous.
I'd think letting the stoker have independent control of one of the brakes was the reason for the second brake bridge and cable routing. In that case it would be operated off a regular brake lever on the stoker's bars. As mentioned, this doesn't help with the usual tandem brake problem in mountains - excessive heat buildup in the rims.
BTW, when our daughter was my stoker (first with a kid-back setup and later with normal cranks) she quickly discovered that the open run of cable along the top tube gave her effective control of the rear brake even without use of a brake lever.