Well, I once had an SA 5-speed hub with the 2 toggle chains, on a camping trip the huge plastic lever (it wasn't like an SLS5N, it was an old, bulky thing which if I remember, worked almost like an early STI shifter, with a push up to change up, and a pull down to change down, and a big turning indicator in the middle) just exploded...no replacement levers available easily, so managed to concoct a system with 2 3-speed shifters, one operating each toggle chain, and certain combinations of positions gave certain gears.
Made for an interesting ride home, glad it was only about 40 miles. Now THAT'S double-shifting!