Today I decided to give XRandR 1.2 a second try, using Debian unstable’s Xorg 7.3 and the experimental radeon driver.
Result: I can get a 3360×1050 desktop with 2 heads. Thats a bit of a surprise as I currently run Xorg 7.2 with a 3080×1050. So XRandR 1.2 decided to let my CRT run at 1680×1050 (after giving it an apropriate modeline for that of course).
If I try to get a 3080×1050 desktop XRandR does tell me that the maximum size of the virtual desktop is that, but it actually still creates a 3360×1050 desktop – no idea why. Also X11 decides to set a DPI setting of 96 which is completely wrong for my setup, proper would be something like 130 dpi as it is with Xorg 7.2.
More serious though is that KDE3 somehow deactivates the second screen again when starting up. And unfortunately I’m currently not able to re-activate it again without X11 going completely blank. The same happens when starting XFCE4 session (though it might happen there due to it starting the kde-stuff like kdeinit).
On the bright side: KDE4 started up perfectly fine on that desktop, unfortunately its not a bit faster so I can’t use it as daily desktop
So if anybody has an idea why kde3 screws my second head, how to get a 3080×1050 desktop or even just how to tell Xorg the right dpi-resolution I’d appreciate a comment.