Alright, Radeon 8500 remix. Last supporting Fglrx driver was released back in August 2006, and that one generally isn't compatible with today's X.org and kernel versions.
So, the driver that you'd be wanting to use is the X.org Ati driver.
I'd start here then : http://wiki.x.org/wiki/radeon
The Radeon wiki points to the RandR 1.2 documentation page on setting up dual head support... http://wiki.x.org/wiki/Projects/XRandR
If you are using a Debian based system, this looks to be the correct set of data to configure a dual head system : http://wiki.debian.org/XStrikeForce/HowToRandR12