My son and I pulled together a ss with a double crank up front, FD and tension-er. So, now it's both a fast ss flatlander and headwind/hill climber. It actually worked out great and feels like a pure ss.

Edit: Now that I think about it, we used an old 6 speed RD, locked in the correct chain-line position, as a tensioner vs. one of the new tensioner components. A cheaper alternative.