I have an additional monitor at home and at work, but they have different resolutions. That was no problem for Ubuntu. I recently set up a new laptop with Xubuntu, and now the second monitor is always mirrored. I solved it by making a few bash scripts with xrandr commands to set the correct monitor settings. So the first thing I do is running home_screen.sh or office_screen.sh after I boot my system. I also have laptop_only_screen.sh and beamer.sh. It works, but still... Can someone explain the different behaviors of Ubuntu and Xubuntu? By the way, my old laptop had a VGA port, the new one has HDMI. Could that be an explanation for the different behavior instead of switching to Xubuntu?
Go to Settings > Display and check the box that says "Configure new displays when connected". Now when you plug a new monitor in, it should pop up a dialog box for selecting the display mode. If this does not work, check to make sure that you are using the best driver for your graphics card. (Settings > Additional Drivers)