"Photoshop currently doesn't take advantage of more than one graphics processor. Using two graphics cards does not enhance Photoshop's performance.
Multiple graphics cards with conflicting drivers can cause problems with graphics processor accelerated features in Photoshop.
For best results, connect two (or more) monitors into one graphics card.
If you have to use more than one graphics card, make sure that they are the same make and model. Otherwise, crashes and other problems can occur in Photoshop."