Another fact :
as long as the two monitors have the same resolution & refresh rate the card should drop the clocks and voltage to IDLE state.
So no 3D mode.
Also I have found out that the new drivers should be used.
Another thing is that the cards never should run on full speed in the latest 4xx (and up) series , when in 2 monitor mode in idle.
They should run around something called "middle state" (something between IDLE and 3D mode).
Conclusion :
Use latest drivers.
Use same resolution and refresh rates on both monitors !
If nothing works from these try to connect the second monitor via HDMI,MiniHDMI or DisplayPort (whichone you have) , just not the DVI.
More Information :
As some point last year, something changed either with ATI's UVD drivers or with Microsoft's media foundation codecs/media center. I used to be able to play a game and watch tv simultaneously . Something changed, and now when trying to do those activities together my (at the time) 5850 would run at 400MHZ UVD clocks instead of 725MHz 3D clocks, making both the game and tv stutter horribly. My 5770 and 6950 exhibit this same behavior, as I assume all AMD cards do.
I have 6950 and three identical Samsung 1080p monitors with DVI/DP/VGA. Activating a second monitor (with same connection, same res/refresh) causes the clocks to jump to an intermediate state: default 450/1250 (not UVD 500/1250, or any value in the BIOS). Tweaking clocks shows the 1250 is linked to full-3D memory clock, and 450 is a function of the same value - e.g. dropping full-3D memory clock to 800 gets 300/800 multimonitor clocks, and down to 750 gets 250/750 multimonitor clocks.
Like Ive said in other threads, my GTX470 idles at 51/101/135MHz (core/shader/memory) after tinkering around with res/refresh rate. Since I use two identical monitors, it was easy to get the timings the same, and voila~ I dont need a secondary card anymore (which initially provided a bandaid solution).
So this does not work for AMD cards?
So if we speak about Radeons , they are worse in this case than NVIDIA.
NVIDIA needs to be synced on both monitors then it shoudl work correctly, but you will need two identical monitors (same resolution and refresh rate)!