What I've got is an ATI radeon 9600 m10.
I've tried the fglrx ati packages to no avail.
I am sure the card is capable of dual heads but I am having problems
writing a proper xorg.conf and also the installation shell script with
the ati package just doesn't seem to be modprobing the ati driver into
kernel space at boot-time.
I had this problem about four years ago with a laptop with an nvidia
card in it... I waited about six months and the nvidia support got
better... I don't want to wait for ati's linux support... they are after
all doing the xbox360 so my guess is they will be a little behind the
curve on things linux in the near future.