Um...almost forgot about the axle "keys"
This dual motor set up will need (4) "keys" in the axle

...(1) for the brake drum
...(1) for the drive wheel
...& (2) for the driven sprockets

They are new but, sometimes you gotta "clean" 'em up a bit
...just sliding them around on a piece of sandpaper usually works pretty good
* I always try to "knock off" the corners
...so, there are no sharp corners to get caught on stuff (like axle, key slot imperfections)

Speaking of key slot imperfections, I've noticed (& had to deal with) many key slot imperfections on axles & in the components too
...where the "key" just won't quite fit (or fully seat) in the slot

Close up pic

A metal file with a nice square profile usually does a good job of "cleaning this situation up"

Close up pic

After a little bit of "cleaning" it should "seat" down in the grove (on the component) nice-n-flat

...& in the slot in the axle too

...& then, slide together & look like this
* See how that "key" works to mechanically "lock" the axle with the component
...so, they rotate together
