First, go to the bios and set integrated to primary. Then you can plug it.

But you'll need a dummy plug to oc, or control the fan.

This is my setup. I use 2 monitors and the 3rd one is set to the lowest resolution. That's one amd card with the dummy plug. I put it there so I won't be accidentally move my mouse there.

http://i.imgur.com/KzwUArM.png