As I mentioned before, the jackshaft assembly includes 2 freewheels.
The inner freewheel and its sprocket will be connected to the rear wheel cassette by the main chain.
The outer freewheel and its sprocket will be connected to the chainring by a short chain.
This setup was inspired by the M55 bike and by builds I saw here on ES.
The parts used for the jackshaft assembly (top left to bottom right):
Jackshaft, inner bearing, inner freewheel adapter, inner freewheel, inner sprocket
spacer, outer freewheel adapter, support bearing, outer freewheel, outer sprocket
After installing the inner bearing, the inner freewheel adapter is clamped to the jackshaft by 2 setscrew 90 degrees apart.
The rest of the parts are then installed.