You *can* do this, however, if:
1) You use an NVIDIA/Matrox card and set it to "span" mode (I'm not sure if the latest ATI cards support this or not). This gives you one big desktop at 2560x1024, and you can use just one overlay surface to cover both.
2) You use a video player that can render in a software mode (or most methods other than overlay surfaces). Then you can put the window wherever you want, even if you don't use span mode.
The same thing applies to 3D apps. But like Peter said, you will have a break right in the middle of the screen.