220v on S2 makes using the device with parallel-connected mechanical wall switches much simpler. Simply connect the electrical phase output directly to S2.
It doesnât work this way, sorry. How supposedly would this 3.3V be added to the AC mains voltage? Any ideas? Enlighten us, please 
Did you measure the Sonoff Dual R3 voltages with a voltmeter? S1/S2 vs LineIN; S1/S2 vs Neutral?
Have a look: Sonoff DUALR3 has a hidden secret - NotEnoughTech
I think the internal A/DC Converter (power supply) of the dual R3 is non isolated, as such the DC ground is connected to LineIn. Unplug the device form the mains, open the open it and measure the resistence with an ohm meter between LineIN and S1
I am familiar with this publication. I will not write about the poor methodology again because I have already addressed this in an email sent to the author.
Going back to your thesis, it doesnât mention âaddingâ 3.3V to the grid voltage. Iâd like to point out that it is impossible to add low-voltage DC to mains AC voltage and get a total of 233.3V, if thatâs what you intended to communicate. Sorry, but this is completely out of touch.
By the way, I dare to point out that in this thread, we are discussing a completely different Sonoff device. Your observations are not really related to this one.
@Ward I suggest closing the thread as the conversation has gone astray; it contributes nothing supported by facts and contains potentially dangerous ideas.