1. Type xrandr or xrandr -q in terminal to dispaly information of all available output device. Then you can use the following commands to set dual display.

   xrandr --output VGA1 --auto 
   xrandr --output VGA1 --same-as LVDS1 --auto
   xrandr --output VGA1 --mode 1024x800
   xrandr --output VGA1 --off
   

where VGA1 and LVDS1 should be replaced by with appropriate device names.

1. When use multi-monitors in Linux, you need to have close resolutions in order for same (mirorr) display to work well.