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its very easy to do, my car is wired to function the way you are asking for.
grab the high-speed (PS) fan plug and put some quick-disconnect terminals in the plug. run these wires to the coil posts of a 40A relay. run a fused 10-14 awg wire from the battery to the power side of the relay and then run a wire from the output post to the fan motors +, and another wire from the motors - to the chassis of the car.
that way the factory fan circuit only sees ~150mA through it and both fans are slaved to the high-speed function.
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