You should be able to play video on either display, regardless of primary/secondary. You may be limited to a single video window, but I have two monitors and both of them can play video. Since you're using a TV, you may have a setting that tells the PC to display a current-playing video on the TV fullscreen while it's open in a window on the primary monitor (my old graphics card did that, where any time I opened a video in Windows Media Player, it would be fullscreen on the TV and windowed on the monitor). You should be able to turn this off in your graphics card's control panel.
