On my 126 car, they both come on. I have a low and a high speed. The later engages when temps exceed 102C. If your fans sit side by side, it wouldn't make much sense for just one to run. Both side-by-side fans would cover the spans of the radiator.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.