The T420, of course, can support two simultaneous monitors (the laptop screen plus another connected to the DVI or VGA port).

How do I enable this with Ubuntu/XUbuntu? It will only let me choose one monitor or the other. There is a cloning option available if I use Fn+F5, but that doesn't seem to work (at least when testing with VGA).

Is this a driver limitation, or XUbuntu limitation, or is there something else I need to do to enable this?

Thanks!

I know this is quite old, but in case you haven't figured this out yet...

Is this an optimus enabled machine, and if so, are you using the Nvidia drivers?

If so, you should change the monitor settings using the nvidia-settings utility - you can launch it from the command line if you can't find it in the GNOME / Xubuntu menus.

If not, you should be able to use the in-built GNOME Displays utility to change your settings in GNOME - I don't know what Xubuntu provides here. If that doesn't work, have a play with the xrandr command from the command line - it will give you feedback as to what's going wrong so you can hunt down the problem.

You can get more info on how xrandr works by running "man xrandr" from the command line, or google it.

(careful switching your primary screen off as you won't be able to see what you're doing! - I run these in scripts, so it works)
Switching to external monitors:

Switch back to inbuilt display:

It's worth pointing out that LVDS1 is my inbuilt display and HDMI2 is my DisplayPort port (no idea why the labelling is off like that).

Thanks...I will play around with xrandr.

I don't have the Nvidia graphics, fyi.

The XUbuntu GUI tools seem like they should support a dual-monitor configuration, similar to how graphical multi-monitor tools work on other window managers & OS's. I don't know if it is a matter of the GUI tool not knowing that the underlying driver/hardware is capable, or the driver itself is not capable.

I had a lot of troubles with both Ubuntu and Xubuntu and dual monitors.
I will recommend Kubuntu. It seems KDE excels both Xfce and Unity when talking bout dual monitors.
I will recommend trying with different cable( though it sounds strange), this helped me force Xubuntu properly recognize my TV resolutions...