Some of the popular desktop environments make it really easy to use two monitors. Some even automatically detect and arrange them for you. However, Openbox doesn’t do this (it’s a window manager, anyway, not a DE) out of the box. And I don’t like to install heavy apps to arrange my displays when I can do it quickly with Xrandr from a terminal emulator.
First, type xrandr in a terminal. This will display the detected monitors and their resolution.
Now, you can enter a command to adjust the resolution and layout of your displays: xrandr –auto –output VGA-1 –mode 1440×900 –right-of LVDS-1
That command is for the external monitor I usually have to the right of my laptop. VGA-1 is the external monitor and LVDS-1 is my laptop’s built in monitor. This command tells xrandr to arrange the external monitor to the right and set the resolution to 1440×900.
I then use this command: xrandr –auto –output LVDS-1 –mode 1280×800 –left-of VGA-1 to set the resolution of my internal display. I usually add these two commands to openbox’s autostart script (/etc/xdg/openbox/autostart) so that Xrandr sets my displays properly when openbox starts up.