I just switched to xfce4 from Cinnamon and the first thing I needed was dual monitor support. From what I can gather reading the forums, newer versions of xfce4 have dual monitor support included.
Unfortunately, the version I installed from apt-get doesn't.
$ sudo apt-get install arandr
Now from a terminal:
You'll get something like this:
Notice my laptop display (LVDS1) and my monitor (VGA1) are superimposed on top of each other, and this is exactly how my display looks right now: the larger monitor has the most content, and the smaller laptop display shows the portion of the content as represented by the rectangle above.
To get dual monitor support just drag one of the rectangles to where you want it, i.e.:
Now click the green checkmark at the upper left.
In a few moments you should be enjoying dual displays.