This question was closed as a duplicate assuming the OP wanted to use multiple monitors, and it was edited to clarify that the goal was a hybrid-crossfire type setup but with Nvidia hardware. Can this be reopened? Or, alternately, is it now a duplicate of Using 2 different graphics cards?
Using onboard graphics card and dedicated graphics card for mixed performance is definitely NOT a duplicate of Using 2 different graphics cards, as what's asked in the lather is (still) unclear to me.
The first one is about increased performance, while How can I enable onboard graphics AND dedicated card simultaneously? is more about using several monitors at once. So the reason for closing the first question is highly debatable.
I would vote for re-opening the question.