You also have to setup a forced detected screen in Windows. That will make the GPU active. It's all a bit funky sometimes, but it does work, and you don't have to plug anything into the onboard stuff.
I currently have this setup running on 3 PCs and a Surface Pro. 2 PCs and the Surface run Windows 8.1 and the last PC runs Windows 7 (it is also the main streaming PC that contains 2x HDMI input cards).
If you want some help and that doesn't do the trick feel free to post in here and I'll drop some contact info.