if you use gdm, you can start another X server by adding

[servers]
# start a second Xsession on :1
1=Standard

to /etc/gdm/gdm.conf.

Not sure if this will start it on a different screen, maybe you'd need some more configuration.

However, if you're trying to do this with fglrx it won't work because for some unknown reason this rather popular functionality is missing. You'll have to use radeon or radeonhd instead. (Perhaps they secretly want to force everyone to use the opensource drivers, so omit important stuff from fglrx?)