When I ran into this issue with dreamscene and multipla video adapters (I have Nvidia 275 GTX as my primary and a Nvidia 8800 to use for my PhysX) I simply switched over to running VLC Media Player, now I can run seperate videos in full screen on each of my 3 monitors, granted it may not be as elegant as other ways, but I really enjoy having 3 different videos playing across my monitors and with VLC the videos stay full screen even when I drag other windows on top of them so sort of making it like they are the background desktop in a way.
The only time that I remember that it's not really my desktop background (I don't play it on my primary monitor as that holds all my desktop icons etc) is when I mouse over one of the monitors and get the little control panel that pops up to let me control the video, sound etc at the bottom of the screen. As I said, it's not as elegant as doing it with deskscapes or dreamscenes with one video adapter but it's suiting my desires very well right now.
And of course, let's not forget that running multiple copies (3 at times but more often just one or two) of VLC Media Player will use memory and CPU power to do so, which means one needs to have a good CPU and amount of memory to do so. I just built a new computer using the X4 Phenom II 965 BE with 8GB of RAM so I don't even notice when 3 copies are running videos (one on each monitor) during normal computer use. I do turn them all off when playing a game as theres little point to trying to run all the videos and a game at the same time in my opinion. Like I said again, not the most elegant solution but works great for what I want... Simply to be able to have seperate videos of my dogs on two of my monitors 24/7 since one of them is dying of cancer and it makes me feel like he's in the room here with me due to the video of him well... being in the room with me.
