When I use dual displays watching YouTube video, it can not switch to full screen mode correctly. I assume it is because YouTube flash client calculate width and height based on the total of dual displays, and render the video into one display. So it is only displayed in 1/4 size of one display (my two displays are of same resolution).
I have tested how videos from 163.com and sina.com are rendered in full screen mode with dual displays. I find both of them are fine. So it eliminates the possibility that bug is in Flash Player.
BTW, I did not find a way to stretch the video into two displays.