Setting up dual monitors in xorg.conf

I have a dual-monitor setup where the second (smaller) monitor extends the desktop of the first one.

This works perfectly fine using the KDE monitor settings/XRandr, but Xorg's default behaviour is to clone the first monitor and setting it to a lower resolution, causing two modesets: One from the correct resolutions (set by radeon while booting) to the cloned view on the KDM login screen, and a second one after the login.

Now I'd like to have the correct setup in xorg.conf. As far as I know, I have to enable the merged framebuffer as described here, however I do not know how a complete, working configuration should look like.

So far, I managed to disable the unnecessary modesetting, but the second screen is still cloning (and now cropping) the frist one:

Section "Screen"        Identifier "First Screen"        DefaultDepth 24        SubSection "Display"                Depth 24                Modes "1920x1080"        EndSubSectionEndSectionSection "Screen"        Identifier "Second Screen"        DefaultDepth 24        SubSection "Display"                Depth 24                Modes "1280x1024"        EndSubSectionEndSectionSection "Device"        Identifier "ATI"        Driver "radeon"        Option "MergedFB""true"        Option "MergedNonRectangular""true"        Option "CRT2Position""RightOf"        Option "MetaModes""1920x1080-1280x1024"EndSection

I am aware that I have to specify Virtual 3200 1080 somewhere in a "Display" SubSection. I tried putting it in both of the existing ones, in an additional SubSection, and in an additional "Screen" Section, but none proved successful.