The software solution using scenes works well but there is the limit of 21 scenes so the limit of 10 switches in a whole house. I reached this limit and I still missing the 3 bedrooms with 9 switches.
@Srdan: That would work but would require new wiring installed in the walls. It would be best to have a way of retrofitting smart switches that uses traditional wiring in the walls for 2-way switches or bypasses the wiring requirement all together.
Why not connect all N of the switches parallelly to the light and still use scenes? This way, all of the switches would work if there is no WiFi...
Are you from Czech Republic? If yes, can you send me your mail? :-)
to Nevy: Sonoff doesnt work with two wire. I have only two switches.
One more thing... You will need an always "hot" wire and a neutral in the boxes to power your wifi switches. If you replace the last switch on the right in the diagram with the "master" switch and tie the black wires together on the first two boxes then you are good. The black wire will always be "hot" in all the boxes. On the other hand if you want to put the "master" switch in box one or two from the left you could use the existing and otherwise unused red wire as an always "hot" for the dummy switches.
If you are doing a retrofit where 3 and 4 way switches are being replaced you would need to make a path to the light bulb in the boxes such that one switch was breaking the circuit and the other boxes were just hard wired together. In the diagram below the red wires would not be used and the black wires would be removed from the switches and tied together with of course the exception that in one box your wifi switch would actually be breaking the circuit of the black wires. In other words there is only one real switch and the others control it with scenes.