Search under Xinerama...I've never used it myself, but I'm sure that you can find the answer on a Mandrake mailing list.
edit: I couldn't resist...see:
http://www.rage3d.com/content/articles/atilinuxhowto/
http://archives.mandrakelinux.com/ne...5/msg00425.php
http://www.linuxquestions.org/questi...003/04/4/56775
The type of driver that you use is going to help determine some of the config info. Are you using kernel drivers or ATI's binaries?
Yet another edit: I have an ATI card in my (linux-only) laptop and it is the only ugly point in an otherwise excellent machine. Until ATI begins creating decent linux drivers for it's hardware, I can't recommend that any linux user purchase one of their products.