Hello Christian,
I am running dual SPAL fans, with the original wiring, without any problem.
BonS already linked my story.
Both fans are turned on by the thermostat switch, or my override manual switch :
both switches are just grounding the negative wire of the fans.
tip : to be able to mount 2 fans, and keep the radiator cover from hitting the fender, I found it easier to use support brackets with a 90° bend, instead of the straight brackets that came with the kit.
pictures here:
Anthony