Two screens (:0.0 and :0.1) and gnome-shell

I can setup my displays (3) using nvidia-settings to have two as screen 0 (DISPLAY=:0.0) and one as screen 1 (DISPLAY=:0.1), but the 2nd screen (:0.1) is not managed by the window manager (gnome-shell).

I can start an xterm on screen 1 (DISPLAY=:0.1 xterm), but that new window, that shows up on the monitor assigned to screen 1, is not accepting any input, cannot be resized, etc… screen 1 is not managed by gnome-shell.

How do I (re-)start gnome-shell to manage both screens???