I have two trucks with the dual SPAL fan setups in them. The 84 has a temperture switch in the intake that feeds the control relay that turns the fans on. I fed the circuit from that to two heavy duty fan relays so that I don't overload the contro relay switch. When the tempo switch closes, both fans turn on. The 91 Silverado has the same dual fans, but the controller is a SPAL PWM fan switch. It is tapped into the temperture switch that feeds the truck's computer and turns the first fan on at one temperture and the second fan on when the temperture gets to high. This is the best setup so far, but is a little pricey. I have purchased another one of these switches for the 84 truck so that I can control the electrical load on my charging system.
The temperture probes that fit into the radiator fins or into the radiator return hose are limited on their abilaity and can be improved upon greatly by looking into other setups.
Check out BE COOL
www.becool.com for temperture switches or Centech
www.centechwire.com .