Single levers for both brakes has always been a fairly common practice for the handicapped and for tandems with caliper brakes and a drum or disc. Dia Compe makes a special lever and cables, and there are splitters out there.
The problem with 2 cable brakes is that they can't be proportioned and need to be kept carefully adjusted so both work together. It's much more practical for hydraulics which can be proportioned (4 brakes, 1 pedal in cars) but I don't know of any dual cylinder levers or line splitters for bikes, though there might be, since it's not something I keep up on.
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