honestly, reading what Derale offers... aside the PWM controlers, the regular relay system they offer are not more than what I made on diagram but single stage... with another difference, they use a positive signal to trigger the relays and I used negative ( ground ) signal to trigger them throught the temp sensor/switch, which in any case IMHO is more versatile and easier to wire up. No need for wire from ignition system ( neither diodes ) except if you want to keep fans off when key is off. Modern cars use to keep fans on with key off

and they have the option of the probe system if you don't want to insert the sensor into the water system, but they offer single stage sensors anyway. You could get a probe and try to fit them with adaptors on to a dual stage temp sensor/switch too.

You can also use 2 adjustable thermostat switches with the probe to trigger the relays instead the dual stage temp sensor/switch.