Setting up an ati radeo card for dual screens in xorg requires editing the xorg.conf file, but is not very difficult. You will need most of the following options in the Device section of your xorg.conf file:

# Where is the second display located?
        Option      "CRT2Position" "RightOf"

# This defines mode combinations, this line assumes the 'left' display is has a 16/10 aspect ratio and the right one is a normal 4/3, adjust as needed.
        Option "MetaModes" "1680x1050-1400x1050 1280x1024-1280x1024 1280x960-1280x960 1680x1050-800x600" 

# enable if your displays will use different vertical resolutions
        Option "MergedNonRectangular" "True"

# Disable to prevent screen corruption with merged framebuffer
        Option "EnablePageFlip" "False"

# which display is display 0? defaults to the most left display
        Option "MergedXineramaCRT2IsScreen0" "False"

# In case ddc doesn't configure the second display correctly..
        #Option "DDCMode" "Off"
# so we set some vrefresh
        #Option "CRT2VRefresh" "50.0-150.0"
# and hsync
        #Option "CRT2Hsync" "30.0-120.0"

Now, given that your displays are detected, all should work and you should have a dual screen setup.