VR Zone explains how to voltage mod the NVIDIA 7950 GX2 graphics card:
First, let's take a look at the GPU Core voltage spots. Even though the primary and secondary boards on a single 7950GX2 are not exactly identical, the mod and voltage read points are exactly the same. You will need to dismantle the twin PCB card to reach the inner board. In any case you should be switching to some better cooling solution when you go voltage modding so you'd have to separate the 2 cards. However, do not test run the 2 PCB half separately!