Godot doesn't support fullscreening on multiple monitors at one, but you can use third-party utilities for this as long as the window is marked as resizable (which is the default).
Thanks for the answer, can you advise such an utility? Especially for the linux platform?
I don't know of any utilities for this on Linux. In general, high-end gaming utilities tend to be difficult to come by on Linux (even though this is slowly changing) :(