I just installed Mint 15 XFCE for the first time today and found your post while looking for a solution to exact same problem. I had many issues on the older PC that I'm using when installing the Nvidia drivers under Mint 15 Cinnimon so switched to XFCE and everything seems to be more stable. I ran into the same issue as you explain and wanted to stick with the standard driver. Anyway enough preamble. There is a pretty simple way to do what you want just using the standard Mint XFCE Installed software: For example to get the second monitor to be independent and to the right of the 1st monitor:
1 - Turn on and enable both monitors using Settings => Display (You can also do this using just the Settings Editor below but Display method is easier for basic enabling and setup of display attributes)
2 - Then select Settings => Settings Editor => "displays" Channel
3 - Set the second (right) Monitor -> position -> "X" Property to the number of pixels you have set for the X resolution of your first (left) monitor and SAVE. For example in my case I have two old 19" LCD 4:3 monitors set to max resolution 1280x1024, so I set the Position->X property integer value to "1280"
4 - logout and log back in so that the display setting changes are applied to your session.
presto, everything works as I want and no additional software or drivers needed.
for more detail with images.