On my Spal controller, I just used the factory LS1 sending unit to trigger it. Then just program it from there....I have the low speed for the first fan come on at around 185°, and full speed for it, and trigger for the second fan (second fan is relay controlled and not variable speed like the primary fan) to come on at 210°ish.
It also has an input for an a/c trigger...I used a trianary switch when I did the air con setup in the car and when turned on it kicks automatically kicks the second cooling fan on as well.