I don’t have a sonoff, but they are based off the ESP8266 which I use a lot for my own custom switches and you can do this if you don’t mind a little hacking.
From what I’ve seen you should be able to solder a wire onto the sonoff and with probably a slight modification to the custom software that is available make it so the existing rocker switches are picked up by the sonoff as a digital input.
From there you just toggle the light state if the rocker switch is flipped and have homeassistant toggle the other light as well.
He put one in the box, but he is using a momentary switch. This just gives you an idea of what you need to do.