I set up this topic to warn fellow makers. In case someone has the fancy idea I had, to drive two Sonoff Minis from a euro-style 2-gang switch which usually shares one live wire to two wires going to the light fixtures like the one below, then just don’t do it.
Essentialy, I bridged S1 and S1 in L and and connect one S2 to 1 and the other S2 to 2, following the wall switch above.
Well… what do you know.
What part of the wiring instructions for the sonoff mini did you not read ? I’d say all of it 
The confidence of installing 10 minis and 8 basic-power relay combos in just two days, made me think I knew better than the instructions. 
Edit: BTW, the instructions only mention the two switch, one mini case. Not mine.
So that I may kill any dumb ideas in their infancy;
The S1s are Ground pins of the low voltage system and isolated from mains. If you first tie S1 of two minis, together, there should be no risk as you have connected just two grounds.
Now, when you flip the switch, you toggle between S2 of mini1 and S2 of mini2, which are GPIO 14 pulled high. They are happy to receive ground.
What went wrong?
