The simplest way Basedown on your basic design, is to use freewheel hubs in both wheels. This has 2 benefits.
1- if both wheels freewheel, you have normal rim drive without engaging the pedal drive train.
2- you have a differential, of sorts since the outer wheel can overrun the inner on turns,
The drawback is that rolling backward is problematic.
The other option is a fluid or friction clutch in each wheel that allows some slippage under torque loads. This will solve the turning problem, but will probably slip a bit during acceleration.
The most perfect, though over engineered solution would be magnetic clutches for both wheels. These would be normally engaged, but either would automatically disengage then that brake is aplied. Of course, both would be disengaged for normal rim drive use.
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