Have you tried the aticonfig tool from the terminal?
Originally Posted by gators38
will tell you what outputs your card has available and what devices it recognizes. "lvds" is your laptop's internal display, "crt1" is the external RGB monitor port (it will be "tdms1" if the external port is DVI) and "tv" for the SVHS or composite TV-out.
sudo aticonfig --query-monitor
should do the trick to enabling the TV as well as your laptop's internal display. If not,
sudo aticonfig --enable-monitor=lvds,tv
ought to do it.
sudo aticonfig --force-monitor=lvds,tv