I tried Xdmx a few months ago. It failed pretty badly for me, so I blogged about it ( http://idupree.dreamwidth.org/2151.html ; choice quote: "Xdmx is broken in various different ways for various different people on the Web. Xdmx isn't very well maintained. It doesn't support XInput2 [1], it relies on Xinerama which is somewhat maintained but whose architecture needs improvement [2], it may be incompatible with Composite [3], it just plain segfaults for a lot of people [4], and is generally unloved [5]." — citations in blog post.)
