Thanks for your quick answers chaps, I think you're both right.
The FWC1 sends signal to Captain's MW/MC pushbuttons upper bulbs (i.e. "Master" in the MW and "Master" in the MC), but also sends signal to FO's pushbttons lower bulbs ("Warn" in MW and "Caut" in MC).
The FWC does the opposite.
Where can I find this information in an Airbus documentation please ?