Dual monitor settup with XFCE 4.8?

I have googled this to death, and have not a solution that works.

I am using Debian 7.6.

I have two monitors. One VGA, the other DVI. I want to use the two monitors as one extended desktop - not reflecting one another.

I had no trouble doing this on Salix, with MATE. With XFCE, it seems impossible.