only one will kick in, even with AC on.
When it gets REALLY FKN hot then the second kicks in as well to give the engine one heck of a blast of cool air. I cant remember the temp ranges but 108 sound about right for the first one to kick in.
Ive noticed mine takes a thrashing before it'll kick in. Unless ive been driving it just wont kick in at idle, say parked in the driveway, either that or I dont have the patience to wait for it too. But if you drive it and get heat into the engine you'll hear it kick in or not, the guage goes up a bit before it kicks in then watch it fall.