He's trying to separate the two so he can run a full screen game on the primary while something else is being seen on the monitor that was added on. The only option for that would be a program like UltraMon or DisplayFusion where you can extend one desktop onto the external display showing a separate app running there while suffering with the pc game running in a windowed mode not full screen!
In order to move the mouse off of the game to click on anything else like opening a browser window even the game will either stall or have to be paused in a windowed mode. Some Steam driven titles however won't stall but seem to slip into a temp hibernation if you are able to recover them afterwards to go check the mail or start up an app you have moved over to the second display before starting the game up however!
The next of course to stall the works will be how to mute the pc game to allow sound for any player? Otherwise two sound output sources will be competing for a single sound output source namely the onboard audio chip!
In other words while a game is paused that would somewhat free up sound resources for many but not all players or the game will have to be able to run without sound. In most cases however you wouldn't be able to have it both ways as far as any game running at the same time as a full screen video playback source. Only one could be in use at a time since you would most likely end up with resource conflicts depending on game and players used.