multiple videocards and slow framerate

hello everybody,
i have a question for you.
I have a Mac Pro with two video cards and i need to use it for a multi projection. I need to use 4 projectors.
my graphic cards are ati radeon hd5770. They have 3 outputs so i need to use 2 outputs on the first board and 2 outputs for the second board. When i do a multiscreen app and the screen use both cards the framerate go down near 20 frame to second instead, using only one card, the framerate is solid to 60 frames to second. There is a method to do this in a better way?