What are you using to use dual display? What OS? What drivers? Which card?
With nView and my MSI GF4 Ti4400, I am able to output just about anything fullscreen to both my monitor and a TV at the same time, this includes: dvds (MSI's player), divx (divx playa and media player), and oldschool console emulators... as well as fullscreen OpenGL games (and I imagine directx).
You have to specifically output to two devices (or explicitly output to your tv), it doesn't happen automatically. If you're using any of the last 2-3 releases of the detonator drivers, you should go to Display Properties --> Settings --> Advanced, then click the nView tab. try selecting the clone button, and/or detecting displays and selecting the TV and checking 'make this the primary display.'