According to my W124 ETM both fans should always operate at the same time. The system does have two speeds, low speed activated by AC condensor pressure and a high speed activated by engine coolant temp. If only one fan responds its a fan or wiring problem with power to the fan. All control components are common to both fans.
I'm also puzzled by people worrying about engine temps - the owners manual is quite clear on the subject. Its OK for the engine to see 100C or even 110 to 120C in really adverse conditions.
I have a 91 300D is operates at 85-95C depending on traffic situations and ambient temps. It rarely gets over 100F here in the mid atlantic region. I also have a 400E that operates at approx 110C in stop and go traffic with the A/C on. The car has NEVER overheated and according to MB this is normal.
If its OK with the factory its OK with me.