The GPU's act as a whole, and require a cpu also. You CANNOT run F@H GPU on both cores and expect it to run correctly, even with two GPU's.
F@H is meant to be a donation program, with a competition for the fun of it. If you really want to boost your score so badly stop trying to hack the program, and goto your local government office or library, tell them about the program, and if they use it, ask if they will use your name/team. My school is currently looking into installing F@H on all the computers, and i asked that if they do please put my name in for the credit. Our computers may only be 2ghz celerons, but 400 of them will make a big difference.