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Can a Fibaro Universal Dimmer module control two separate light circuits?
The Fibaro dimmer module can only be wired to one lighting circuit - this is also know as the 'local' load. This is controlled by the dimmer's S1 switch input as shown in this guide - Installing a Fibaro Dimmer
So if you have a 2-gang switch controlling two separate light circuits, you will need two dimmers.
You may not be able to install two dimmers in a single switch back box, but there are a few options:
Note: The dimmer also has a S2 switch input, this can be used to control another Z-Wave device that the dimmer is associated with, but this is not the same as directly controlling two wired 'local' loads.