The solution seemed to be streamline by reinstalling a new version of VirtualBox, I am not sure the issue could have been caused by upgrading Windows home to Windows pro, or playing with virtualization settings, but due to this thing it was not possible also to Enable multiple virtual displays inside guest Virtualbox even when multiple was configured always only a single one was available, by clicking enable as the menu did nothing (the issue elaborated also here https://forums.virtualbox.org/viewtopic.php?f=3&t=93941#p453148 ) nor a workaround by xrandr tool adding and outputing a new resolution to the disconnected monitor. (
I think I may have spoken too soon. I was able to create a "virtual" display by adding a new mode to one of the disconnected displays listed in xrandr using xrandr --addmode VIRTUAL1 1024x768 I then simply enabled the display like you would normally: xrandr --output VIRTUAL1 --mode 1024x768 --left-of HDMI3 Furthermore, combined with x11vnc -clip, I am able to use this to convert my android tablet to a second monitor. The perfomrance is pretty crappy at the moment, unfortunately, but that's whole other area to be explored.