"I wonder if is possible..."
No it is not possible. The Sonoff T1 is a simple 1-way switch. It will not replace a 2-way switch and allow the other 2-way switch to function properly.
No it is not possible.
The Sonoff T1 is not a 2-way switch design, so you are not able to use it to replace one switch (gang) in a room which has the lights operated by 2 separate switches. I assume you know that the T1 also requires a neutral wire, which may not be present in your existing switch box?
You could use one channel of the 4ch Pro to replace a single gang (2 channels for 2 gangs), but that is quite large and would need careful positioning.
The Sonoff T1 is not a 2-way design, so it cannot replace a switch (gang) in a room where the lights are controlled by 2 separate switches.
I assume you know that it also requires a neutral wire, which may not be present in your existing switch box?
One channel of the 4ch Pro can be used to replace a 2-way switch (2 channels for 2 gangs), but that is quite large which presents its own problems.
If you can solder a couple of wires in parallel with one of the buttons, then you can use one channel and replace your wall switches on the 2 ends of the circuit by momentary switches in both places, both connected in parellel with the original Sonoff button.
This way you can turn on and off indistincly from either place, from the app or even google home / alexa. It will keep everything syncronized.
Good luck !
Why Sonoff doesn't build a wall switch with NO & NC contacts and a simple current sensor to detect the on state of the load is beyond me. Users could replace both regular and 2-way switches with it. They already have the parts and the engineering done and would sell them by the boatload.
It would be nice, but I don't think it can be done with a Triac, design which is used in their smart switches. To operate properly it would need a relay, which then has issues of space and the "latched on" state of the coil. If there was a power cut, off/on, the lights may then be incorrectly powered on ...or some extra logic control required.
I'm guessing that's why smart switch manufacturers are avoiding that type of device.
They could easily use a relay instead of a triac. The smart light switches I am using are Oittm and they use a relay. The relays in the basic and pow are rated at 100% duty cycle so always on isn't an issue and Sonoff incorporates a "Keep" setting which will bring their devices back up in the same state they were in when power was lost. And hey, if they wanted to go real fancy a mechanically latched relay/switch could be used. Basically all you need is the circuitry of a Sonoff POW with NO and NC contacts built into a light switch. They already have most of the product developed it just needs to be built into a light switch package.
As a retired engineer I just don't see this as a tough problem to solve and there IS a market for it. Leviton has some devices out but they are expensive and in my opinion not as good a solution as what I am talking about. I am surprised one of the Chinese manufacturers has not put one on the market yet.
EW, do you have a link for the Oittm switch you are using, the ones I have seen are single pole only.
Like all the rest it is SPST. I mentioned it only to illustrate that a relay can be used in a light switch instead of a triac.