I think the patch of freedesk Bug 2597 ,can resolve the bug of dapper 6.06. https://bugs.freedesktop.org/show_bug.cgi?id=2597 I download the src use "apt-get source xserver-xorg-core" , patch it as bug 2597. Now my dualhead is ok. deb-src is more easy to build than CVS from X.org. -- XOrg dualhead regression https://launchpad.net/bugs/36461