First of all, you only have two sources for graphics - integrated and discrete. You won't be able to combine these unless you have an AMD APU, but I doubt that's what you're using. When you connect your discrete gpu, you motherboard will automatically select the card as the primary graphics device and the onboard graphics will not be used.
There is some software called VirtuMVP, which theoretically claims to combine your onboard and discrete graphics for better performance, I've tried it, but I didn't see any improvement.
Also, discrete graphics are always going to be better than integrated for a while.