[47 Fasttoys]

Yes you can, on a DC motor the speed is based on voltage applied, by running 2 motors in series you basically cut the voltage in half. 6 volts to each motor hence 2 speeds, 1/2 speed(6V) and full speed (12V). Your older heater fan works the same way, on the fan speed switch it has resistors on the rear which cut the voltage to the motor 12V, 9V, 6V hence 3 speeds.
Hope it helps.
Rob