I have 2 light switches that control one light.
Is it possible to connect Sonoff ZBmini without adding additional wires?
Or is it possible at all?
Yes, it is possible. You need to give more details about the wiring and the switches. Are they cross switches?
The drawing… my eight-year-old grandson would be delighted 
This is much better, indeed.
As there is no N conductor in both boxes, the only option that does not require alterations to the installation is to use ZBMINI-L2. The device is extremely small in size and will certainly fit even in a shallow box, under the switch. You place ZBMINI-L2 in one box.
You make the connection according to this diagram, remembering that L and N wires must be connected exactly according to the drawing. The designations of the terminals S1 and S2 are important. In the option from the attached drawing it must be the S2 terminal.
Here’s the video tutorial.
- N present in the box (left one and right one) is just not connected to any switch.
- If I place relay to the left box:
- Issue with Line out (line out is on second box only)
- If I place relay to the right box:
- Issue with the line - line changing always
Looks like relay should be on the left box only, but in this case how to make lineout ;(
This is the right diagram I have implemented:
Is the wiring in your home exactly as shown in the diagram? In particular, does the cable connecting the two boxes have four wires? What does it look like in reality?