Theoretical question about double drivetrain (both-sides drive)
After thinking about this for awhile, I think this would work, albeit somewhat clumsily. It would operate similar to a retro-direct, except with no reversing for the second freewheel. You would have to use good quality freewheels to avoid having both of them engaging together and binding.

To answer a commenter Blam:


  Just how would you expect it to magically shift grears? The are both attached to the same axle. 


The two freewheels being engaged in opposite directions would result in only a single freewheel being engaged at a time. He wouldn't be able to 'change' gears, going forward, but would see a different torque profile when slowing down compared to when going forward.