Linux: Use VNC to create a second Screen

I'm planning to use my 7" Android tablet as second screen via VNC.

Sharing my main screen works fine with x11vnc, but that is not what I want.

So I created a second screen using the xorg86-video-dummy driver and share this screen with x11vnc -display :1 which works but I don't get it managed to use xrandr "extending" this screen with the main screen... .

What am I doing wrong or is there a better solution? The driver for the main screen is the cs nvidia one.

I think I have found a solution using xdmx instead of xrandr; I will try this on a virtual machine running two xservers (with dummy devices) plus xdmx as primary xserver.

My first try with two "dummy xservers":

Xorg :2 -config /etc/X11/xorg.conf.dummy & // my "real screen"
Xorg :3 -config /etc/X11/xorg.conf.dummy & // my "vnc screen"
Xdmx :4 +xinerama -display localhost:2 -display localhost:3 &
x11vnc -display :3 -noshm

Works, but now I have to tell slim to work with xserver number 4 and put this all together into a failsafe bash script.