I give up.
The Series-Parallel switching works, but the series mode doesn't offer enough torque to be very useful for acceleration.
Top speed in Parallel is only 18mph.
Top speed in Parallel is 35mph.
And the battery pack is struggling to supply the demanded power.
The bike is too heavy. The motors are too small in wattage for the weight. The batteries don't have enough capacity.
Most of the above issues could be resolved with a larger pack of lithium batteries, but I'm not wiling to spend that kind of money right now.
And I can't afford to work on this project any further for the time being.