A motor and generator are the same thing. An applied voltage to a motor/generator determines how fast it wants to turn. If it is turning faster, it generates power (current), which causes a drag that tries to slow it down. If it is turning slower, it pulls power (opposite direction current) to try to make it go faster. The two wheels (motor/generators) are wired together and will want to be turning the same speed (assuming they have the same Kv). Turning either wheel by hand will generate a current that will drive the other to go the same speed.
This is easier to demonstrate with two simple DC brushed motors, because no other electronics are needed. Just wire two brushed motors together and you will see the exact same thing, where either one being turned will cause the other to turn the same (or opposite if the wires are crossed).