I've only configured dual monitors on intel and nvidia graphics. You may need to install the proprietary ATI driver. See the following article on the Debian Wikihttp://wiki.debian.org/ATIProprietary
You will need the older version of the driver, which can be found in the Lenny repositories. "Catalyst 8.12 (fglrx 8.561)"
If you go this route, I recommend downloading the files from packages.debian.org rather than adding the lenny repository.http://packages.debian.org/lenny/
Hope this helps you get started in the right direction.