Hi Karl you indicated a chain length of 600mm which is nowhere near the length required to go from the bottom 12T 12mm bore sprocket to the upper 28T 8mm sprocket. I got a 1.5m length thinking I would simply split that for the 2 pedal units but discovered that it used approx 925mm to go around both sprockets and distance between the 2 shafts but was quite loose and with the slack would cause the non loaded length of chain when winding to jump off the sprocket teeth. I shortened the chain by 1 outer and 1 inner link so the joiner would match again and was 1 tooth too short impossible to tension and rejoin the chain at all.
A possible solution could be to use a simple 3mm angled aluminium bracket with 2 roller bearings fitted on the vertical face and act like a chain derailleur tensioner just below the upper sprocket. The bracket simply mounts to the existing 2 screws that are used to secure the rear flange bearing. Am making a prototype and w ill post a pic if it once fitted and if it works or not