No the motherboard video will be tied to whatever video chip is utilized in your CPU. Your discrete video card is completely separate from the motherboard video. If you plug in your monitor to your motherboards video connections, you will draw whatever the CPU video chip is capable of, and same with the discrete video card, if you have a monitor plugged in, you will draw whatever it is capable of.
They are 2 completely separate entities, and therefore do not really work together, but they work by themselves. So you'll always want to use your discrete card for graphics intensive work.