Fuji Special Road Racer Retrodirect

Not too hard, but there are some fiddly bits. Use a dicta brand freewheel on the hub, those have threads that go all the way through. Then screw in an old bottom bracket cup to extend the threading, adn the second freewheel goes on the BB cup. Might need some spacers in there also, and I had to respace the hub a bit.

The main challenges are chain tensioning, and chainstay interference. There was only one configuration here (with the smaller freewheel inboard and being driven in reverse) where the chain didn't rub on the chainstay. Classical retrodirect bikes had a very curvy right chainstay to clear the chain.

The Kore chain tensioner works, but it's pretty fiddly (wouldn't clamp on a chainstay that small, hard to make it hold its angle) If I were doing another I might rig up part of a rear derailleur on an e-type derailleur mount off the bottom bracket.

...is to clamp a piece of galvanized electrical conduit underneath the chain tensioner's clamp, as I did on the non-driveside chainstay in this build to spread the torsion from a drum brake arm. If you use a hacksaw or Mototool to slice the conduit into a half-cylinder, you get a few millimeters of spacing; the clamp from the gadget holds the conduit in place.

Did you have to redish the rear wheel once the freewheels were mounted?

I think this design has been around since the early 1900's. Hirondelle (France) made one called the Rétro-Directe that is in Jan Heine's book 'The Golden Age of Handbuilt Bicycles':

"The gear mechanism is remarkably simple: The chain is wound in a figure-eight around two freewheels (of different sizes). One freewheel is engaged when pedaling forward, the other when backpedaling. To change gears, one simply reverses the pedaling motion and pedals backwards."

It goes on to say their top-of-the-line model had a front derailleur that gave it 4 speeds. Interesting design. :)