Honestly, your on a linux distribution with a version of Xorg that is over 5 years old. The chances of getting useful support with such an old version of Xorg is small. All I can really say is that the driver does not support xrandr 1.2, so you'll have to use MergedFB. I suggest checking the 'radeon' man page for information about MergedFB. But, again, bear in mind that the driver has changed a lot in 5 years. It's quite possible that the error about not detecting a secondary monitor is a bug in the driver back then, and has been fixed in newer versions.
Is there any reason why you're returning to Linux by installing such an ancient version?