I've been looking into this as well and best I can tell, the issue is that your window manager (Gnome) uses the
xinerama extension to get hints about monitor locations and uses them to place maximized/fullscreen windows. If you can prevent this, you'll get your wish.
General Options, Display Settings, disable
Detect Outputs and delete any outputs and you should be able to span windows correctly.
Alternately you can try setting your resolution similar to the example. Be sure to remove the example which will be like
The other way involves reconfiguring X to not load the
Note that the
xinerama extension is distinct from the
xinerama feature. You'll see in
/var/log/xorg.0.log if the extension is being loaded, and you can verify that with the command
xdpyinfo | grep XINERAMA.
I don't have the answer for how to prevent the extension from loading, but I was unable to stop this via
xorg.conf using the