The Debian versions of Xorg 6.9 and 7.0 seems to have a problem with the xinerama extension (or at least I couldn't make it work. Which made me turn my head to the MergedFB pseudo-Xinerama option of the radeon driver. Below you can find my configuration that made it work for my setup.
MergedFB in turns out to be a beast of its own and it took me some time to get lucky with the configuration options. What I want is one big desktop composed of two screens with the resolution 1280x1024. In my setup all resolutions up to 1024x768 works fine, but as soon as I go to 1280x1024 X switches to "clone" mode - the same image on both screens.The trick for me was to enable:
Option "MergedDPI" "100 100"
And suddently I have a 2560x1024 desktop
The rest of the details can be found in my xorg.conf.