I had a NVidia 8800GTS, and I ran a dual monitor setup with my LCD monitor and my HDTV. It ran perfectly fine. Now I just got an HD4870 and it wont work. In the control center there is a secondary display but it won't let me enable it at all. I right click then hit enable. It acts like it'll work then it disables again. Please help.
I believe I might have had the same issue with my 4850. I ended up setting the tv as the primary, set the refresh and resolution on that to match the monitor, and then switched the monitor back to primary and it has worked well since. I cant run certain games for some reason (I think its related to refresh rate) but other than that it's fine. This is by no means a professional workaround, I just fiddled with the CCC for a day or two till something worked.
I'm sorry, you said HDTV, so I assumed you were using the other DVI port. If you have one DVI and one Svideo, switch to the other DVI. I had this exact setup with my x1800XT. One monitor worked fine, but as soon as I tried to use the Svideo port, it would "click on" for a split second, then turn off. I found out that when using the "Svideo" port, it turns off the second DVI. Stupid me (and I assume you to) had the one monitor plugged into the second DVI, not the primary. Leave the Svideo cable where it is, move the monitor to the other DVI. Your probably plugged into the secondary.
I have the same kind of problem, but with 2 viewsonic Ve700 (LCD), connected with 2 VGA -> DVI adaptor. perfectly symetric. I try to extend to second screen and it does not work. It did work before i had a system crash and i had to restore the boot sector. since then the second screen does not work, even aftre reinstall of latest radeon 4800 drivers. Any ideas ? At the beginning i had to try permutation until finding a solution but this does not work anymore.