Hi Kristo, in order to have the 2 way switch and the app working at the same time as expected you will need to use 2 TX, only one of them will control the light, the other will be just a "floating" unit connected to no where.
Then you link the 2 TXs using the scene functionallity. That each time one channel goes on on the A panel, you turn on the respective channel un the B panel, the same for tunrinng of and respectively por the other channel. That way you would control the status on the Tx you touch and the other will replicate the status.
For the electrical connectivity you proabaly have inverter switches on each end, with each extreme connected to live and neutral and the middle to the light. So lets say you connect panel "A" on one side normally, that is powering the Sonoff TX with live, neutral and connecting the light on the respective Lx connector. On the other side (B panel), you connect the other TX and power it as usual (live and neutral) but do not connect any charge at all. Then need to connect the wire going to the middle in your switch into the neutral input of the TX. This way you will be carrying Live to your light trough the A panel, and neutral received directly from the other end (B panel side). Take into accounf the B side does not go trough the B panel, it just takes neutral permanently from the input (not the output)
Hope this helps
Here a diagram....