Well, I don't get the real problem:
a) do you want to have the different resolutions /simultaneously/ as your subject depicts
or
b) do you want your X server start with 1920x1200 when no external monitor is attached and 1280x1024 when the external LCD is connected?
In case a) you have some additional choices: different desktops, one big desktop over both displays, cloned display.
In case b), replace the line
Code:
Screen 0 "aticonfig-Screen[0]" 0 0
with
Code:
Screen 0 "aticonfig-Screen[1]" 0 0
Best regards,
pi