Newbie question - Only see one of two GPUs on Macbook Pro

I’ve got a MacBook Pro with the Nvidia 9400 and Nvidia 9600GT. However whenever I look for available GPUs I only ever see the 9400. I’d obviously like to take advantage of the 9600 GT as well. This is what I’m calling:
err = clGetDeviceIDs(NULL, CL_DEVICE_TYPE_GPU, 0, NULL, &numDevices);

Any ideas why the 9600GT is not visible?

Make sure you’re in the high-performance graphics mode. Otherwise the non-integrated GPU will be disabled.