Here is a concept for a roll your own tandem mid drive ebike.

Take one standard tandem, create a brake to hold a standard rear wheel hub motor between the riders, with a single freewheel on the hub motor, with the chain line matched to one of the cogs on these cranks:
https://www.sjscycles.co.uk/cranks/s...558d65&geoc=US

Use standard tandem cranks on the left for the timing chain if synchronized pedaling is desired, or add a third chain ring to the above cranks, with one set of the cranks above in each position for independent pedaling.

When electrified, this will be a 1X drive train, prior to that a 2X is practical.

The largest 104BCD chain ring that I have been able to find is 52T. DECKAS offers narrow-wide chain rings - great for 1X, not so much for 2X.
https://www.ebay.com/itm/DECKAS-Bike...sid=m570.l1313

Hub motors typically spin around 250 to 400 rpm, choose the freewheel size and chain ring size used for the motor to approximately match your max cadence, or a bit higher. There also needs to be a chain tensioner in the system. With normal human compatible gearing, even a 250W hub motor mount mid drive system is a lot of boost.

Only cadence based PAS will work, unless you can engineer your own torque based system. On my bikes, I am not using PAS, I just use the throttle.

EDIT: just found this: Cranks 170mm - Freewheel Crank Set - Sick Bike Parts which allows other options, basically roll you own.