Understand the OS itself allows the acceptance and the Nvidia s/w will provide the latitude you need. You will also need to be sure your PSU is capable of a stable output for both. However, don't overdrive these video cards as they are a mismatch and of different levels other than basic or non-upper level of video. Its best to have very similar video cards and of make for best performance. But, there is nothing for now to stop the above.
I currently hav installed the stock NVIDIA GeForce 9400 GT (512MB memory) and I am wondering if i can install a second card, NVIDIA GeForce 6600 GT (128MB memory). The motherboard supports 2 PCIe slots, one x16 (currently used) and the other x4. Can anyone help me?