A dual-pull lever is the obvious alternative. Probably unobtanium for drops, but the MTB style ones work fine.
Or use hydraulic brakes and split the hose.
It does seem like a slightly odd thing to want to do. For a start, I've no idea how you'd go about balancing two cable brakes on the same wheel.
Independent control would seem a better option, unless there's a really good reason not to want it? Either with a friction shifter on one of the brakes, or off-load it to a lever controlled by the stoker.