only one pinch roller is engaged at a time, depending upon which direction
the tape is traveling?

Yes
so the two flywheels turn in opposite directions?

Yes
How do they get the second flywheel to spin in the opposite direction?

look at the diagram (in the original post). The Belt goes around the 2
flywheels such that the belts are driven in opposite directions.
Maybe that has something to do with the need for a different size?

M

Flywheels are differentdiameters AND capstan spindles (pressed into the
center of the flywheels) are different diameters. The flywheel/spindle
diameters are calculated such that the resulting tape speed is the same for
both, only in opposite directions (one for Side A, one for Side B).

The reason for the differences, it seems, is to help avoid (or minimize)
resonant frequency set up by the spindles turning in their bearings.
Different diameters/speeds = lower resonance.

(op)