Logic Pro 9 using logic pro on mac with 2 video cards

I,m using logic on a new Mac Pro and using all 3 video out for the graphics card. Screen set changes are very slow when we have a movie on the 3d monitor. Get rid of the video, things are fast as always.
We are going to try and put the video onto it's own graphics card. Has anyone got any experience with this? I'd really appreciate knowing before I spend the time trying to do it only to discover it won't work for some reason...
George Leger III