I took a look at your posted youtube.
This is not a good solution for the two switches in the staircase.
I don't understand how you expect this to work in all scenarios.
You basically connected the two switches in series.
Only once the first one (left side, near the power IN) is ON, can you turn ON the second one.
If you try to turn the second one first (right side, the one closer to the light) it will not respond, since it has no power.
the demo looks nice, but it's not a real solution.
So what is real solution?
I don't have a better solution, but I wanted to point out that the youtube clip is not solving it.
People might run and implement it without understanding it fully.
Until ewelink/sonoff don't offer a built in solution, connecting each separately to power linem and writing your own FW will do it...
But I too prefer to get original FW/SW support and not hack it myself.
IF the Sonoffs have relays with both NO and NC contacts, and I don't know if they do, you can wire them as a three way switch. The contacts that are exposed are the common and Normally Open so doing this would require you to bring out a wire from the Normally Closed contact connection on the circuit board yourself.
That said, you will not know from the app whether the light is on unless you compare the state of the two switches. If wired like the above diagram the light would be on when both switches are in the same state. The light would be off if the switches are in a different state. In other words if both switches are on or if both switches are off then the light is burning. If however one switch is on and the other off then the light is not burning. I suppose you could create scenes to accomplish this.
Relay in Sonoff touch is HRS3FNH-S-DC5V-A
I have never paid attention to the circuit board in a regular Sonoff and don't own a touch. I would think the answer to your question is yes although you might have to locate the relay off the board. Any relay with the same coil specifications can be energized by the circuit.
It is true that you can use scenes to replace 3-way and 4-way switches. Essentially have one switch that actually controls the light and all other switches in the scenes control themselves and the master switch such that if any switch is on, all are on and if any switch is off, all are off.
The downside to this approach is that it does not work if the Internet or wifi network is down. When that happens only the one actual switch will control the light by manual action.
I made a suggestion to another manufacturer a while back that they produce a 3-way switch and incorporate a current sensor. This could be used to replace only one switch in a multiple switch situation and leave the other manual toggle switches in place. The state of the light would be known by the app software based on the current load. If the light is burning if would be drawing current. Toggling the wifi controlled 3-way switch would turn it on and off as would the other manual switches in the circuit. All the while the app software would still be able to determine the state of the light bulb.