
for example while testing/tweaking I prefer to use the 9400m for Octane and the 9600GT for display (unfortunately I can't do the inverse...). Once the file setup is ok and I don't need the macbook for other things I can let it render the final with the 9600GT or both cards...
on a side note, the 9400m is 16 CUDA cores, the 9600GT is 32 and the 330m is 48... a GTX480 is 480!