I recently set up a second monitor and then managed to use it to display video whilst still working on my main monitor as usual…
Here are the main settings to add to your xorg.conf file. In the "Screen" Section Add..
Option "MetaModes" "1024×768,1280×800"
Option "TwinView"
Option "TwinViewOrientation" "DFP-0 LeftOf CRT-0"
Option "TwinViewXineramaInfoOrder" "DFP-0, CRT-0"
To find the id's of your monitors look at the /var/log/xorg.0.log file. I reversed the order of the screens using "TwinViewXineramaInfoOrder" and placed CRT-0 to the right of DFP-0 with "TwinViewOrientation".
To play a video file on either screen you specify the "-xineramascreen" parameter with "0" or "1". For example…
$ mplayer -aspect 16:9 -stop-xscreensaver -xineramascreen 0 video-file-1.avi
$ mplayer -aspect 16:9 -stop-xscreensaver -xineramascreen 1 video-file-1.avi