1996 Cadillac Deville Radiator cooling fans

It was my understanding with 2 fans only one runs until A/C is switched on. Well #2 fan don't come on tested ok and replaces the 3 relays on bottom that go to fans replaced temp sending unit in the rear head and checked all fuses. Question is aren't these 2 speed fans and what could cause the #2 fan not to come on with A/c running. Because it make motor run quite hotter then normal.