Is there a way to watch a video fullscreen on 1 screen while doing something else on the other one without loosing the fullscreen ?

This is not only Adobe but every video player. For the Adobe I've found a solution by modifying a .dll file with an hex editor. It work great.

http://www.lifehacker.com.au/2010/06/keep-flash-10-1-in-full-screen-on-a-second-monitor/

wow nice find. Silverlight comes with he option, when you FS with dual monitors it jsut asks you: Want to keep me fullscreen no matter what?" IE netflix.