The resistor [ R15] is only used in the Low Fan circuit..the High Fan circuit does not use the Resistor..allowing full battery voltage to fan motors.

The Low Fan circuit is actually called the Pre-resistor circuit.
So, to answer your question..there are two speeds-- High/Low. And they both have independent circuits [ seperate relays,sesnors, fuses, etc.] They share the same fan motors only
The low fan circuit is triggered by the a/c system high side pressure switch and has the Resistor in series with the fans..that is how low speed is accomplished.
The High fan circuit is triggered by the Engine coolant temp sensor/thermistor @ 105C and is direct battery voltage, no resistor used for that circuit.