[NixOS/xorg] Did anyone succeed with virtual displays?

I am messing around looking for a way to expose a virtual display as a VNC service to use a phone as a secondary display. People suggest to add the VirtualHeads option in the device section. The way I am using is the following snippet:

  services.xserver.deviceSection = ''
    Option "VirtualHeads" "2"

And I expect two VIRTUAL outputs appearing when I run xrandr.

From this I could expose those displays using x11vnc.

I am already using an external monitor.

EDIT 1: My device is a laptop i5 7200U with an Intel Graphics 620. My current workaround is mirroring a terminal session in an external display (old tablet) using gotty and tmux, after it’s running it runs fast enough xD.