Regulate FAN voltage from another potentiometer (Different voltages)
I don't understand your "Last image"... I guess this is your setup:



Both voltage converters share the same ground.

As @Peter said, without knowing the schematic design of your converters, the safest way is to use a dual potentiometer.

If you really want to use single potentiometer, it may be possible to wire it to bias two transistors. Again, without knowing what devices you have it is impossible to tell exactly. To make an idea:







simulate this circuit &ndash; Schematic created using CircuitLab