That setup wil work fine, but I would use two relays. You could use 2 seperate switches with different on/off points if you want to stagger them, or you could just tie the relay coil grounds together and then pass it through on thermo switch. I did a writeup on FA a while back on wiring up altima fans using a fan controller and listed what wire colors a pin locations correspond to high and low speed. I figure this is a good place for them. For low speed operation you would only use one pair of wires on each fan. You could use relays to trigger high and low speed, but I think for simplicitys sake, just stage the fans wired on high speed. I just supplied power to both relay contacts because on a fan contoller, when the car is off the coil ground is open. If using a thermo switch you will want to wire it to ACCY like you have shown, or the fans will run when the car is off and the thermo switch is closed.
