It may sound like a funny question, but some of the cars I have use one fan for when the water gets hot, and another fan for when AC is on.
Onto my point. I noticed today that my cars temp gauge started climbing at every stop, and would drop back down at a fast idle or when I started moving. Naturally I looked to see if the fans were working. One of them is, and one of them is not. Obviously if the car is designed for two fans to run at once, then it's not getting enough airflow at standstill and that causes my car to overheat. But if it is supposed to be that way, that means it's something else.