I am currently the caretaker for a bright orange Gitane Tandem from the early 70s that has one of those two-cable levers on it, and it's a horror.
Pulling that much cable through that much casing, no matter how you try to reduce the friction, requires giant hands and gorilla strength, and I'm either going to completely revamp the braking system on this bike, or sell the bike for something more contemporary.
Personally, I think it's unsafe. With big, modern Kool-stop brake pads on both Mafac cantis, the whole lash-up works, just barely, imho.
Charles "likes brakes that actually stop a bike" Andrews SoCal
PS: yes, this Gitane isn't long enough for the stoker, but someone thoughtfully installed a special high-offset seatpost on this beast, and so, it's rideable after a fashion. Those 70s Tandem makers must have figured everyone had long legs and no torso...