First of all, a clear warning: only qualified personnel, an electrician, may and should intervene in the electrical installation of a house. Mistakes here can be fatal and can even lead to a fire. Damage caused by unauthorized intervention in the electrical installation also means that no insurance will cover the damage.
So this is not a guide to doing it yourself, just an illustration of how it can be done.
We have two ceiling lamps in the living room and to be able to switch them from two doors, there is a changeover switch with two changeover switches on two doors of the room.
Unfortunately, only one switch has the phase conductor L1 and the neutral conductor N. But there are four wires between the switches.
I was then able to make both lamps smart with two smart series switches.
Using the wires between the old changeover switches, the phase conductor L1 and the neutral conductor N can be routed to the other switch.
This means that the right switch can switch both lamps and the left switch can control the right switch as a dummy via automation.
Goal achieved, both lamps can be controlled smartly and like a changeover switch from both doors.