Your confusion is very common with these two different systems.

Fan Systems/Explaination :

The two fans are wired in parellel and ALWAYS work in unison.

There are TWO different circuits ,HIGH FAN and LOW FAN.

Each circuit is INDEPENDANT of each other , EXCEPT, they SHARE the same FAN MOTORS.

There is high fan relay, fuse , sensor circuit.

There is a low fan relay, fuse ,sensor , and dropping resistor [ Referred to as R15/ Pre-resistor.]

The a/c circuit can NEVER trigger a High fan, nor can the coolant temp circuit ever trigger a LOW fan..however , if the coolant circuit calls for high fan, that will over-ride a low fan demand from the a/c low fan circuit, resulting in high fan, regardless of a/c demand.

So, if one looks at a schematic, you will note that each of these independant circuits MEET at a COMMON terminal at/after the dropping resistor. [ the two wire side of the resistor]
That is the point in the circuit where either circuit feeds the FAN MOTORS. That is where they now share the fan motors, regarless of which circuit has a demand signal from it's circuits sensor [ Coolant temp sensor for high fan, a/c pressure sw/sensor for low fan.]
The low fan is accomplished by feeding 12v to the R15 resistor BEFORE this common terminal, thereby dropping the voltage to get a low fan RPM. Whereas , the High fan circuit by-passes thus R!15, allowing a full battery voltage to reach the fan motors, resulting in a HIGH fan RPM....
..so, two different circuits Sharing the same , parellel wired fan motors.
If you have a fan motor turning slower than the other , or not turning at all, the problem is NOT in either high/low fan circuits, it is a faulty FAN MOTOR .