In my 97 X300 at least I think it works like this.
Power is permanently fed to the RH fan and one side of the LH fan is attached to earth.
When the Refrigerant Single Pressure Switch operates at 12 Bar or the Radiator Thermostatic Switch hits 88C the first relay switch opens.
The earth path for the RH fan is diverted THROUGH the LH fan. As the fans are in series the voltage across each is 6V - the circuit voltage of 12V equals the sum of the voltage across each fan. The fans spin slow.
When the second switch opens at 20 Bar or 100C, the RH fan now earths via a separate dedicated point - 12V, full speed. The LH fan receives its own feed of 12V which flows through it to earth, so again 12V.

As my Single Pressure switch has been replaced with a wire loop the fans are always running at the slow setting until the higher pressure or temp is reached.
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Doesn't it change between series and parallel to achieve low speed and high speed?

 

Good point that I probably missed , will require drawing out the 2 different high and low temp from the coolant temp switch . Once those 2 are squared away they can be expanded on using the other mode of AC pressure switches .