Distributed Multihead X is a mainline X.Org Server feature to act as a proxy X11 server to multiple X server back-ends. When going in-step with Xinerama, the multiple X Server back-ends can then be merged into one large virtual screen. While DMX/Xdmx may be interesting on a technical level, it's not used often these days by Linux users and GLX proxy isn't too sufficient for running OpenGL applications in this manner. Other details on DMX can be found from the defunct project site.
While Distributed Multihead X has been a mainline X.Org Server feature, it's being disabled by default for the X.Org Server 1.16 release. With the 1.16 merge window open, Keith Packard made the change of keeping the support present in the code-base but disabling it from being built by default.
Keith wrote in his commit, "It's clearly un-loved, having piles and piles of warnings. If someone wants to fix it up to compile without warnings, we can re-enable it." Keith Packard was recently on an expedition for eliminating X.Org's compiler warnings.