Utility Monitor On The Onboard Graphics Chip: Consequences?

[Strill]

As I understand, some motherboards have onboard graphics chips that can run at the same time as a dedicated graphics card. Are there any performance consequences for this? I've heard that mixing onboard chips and dedicated cards from different manufacturers can cause driver conflict issues, but I was wondering whether I could gain some level of additional performance by putting my utility monitor(s) on the onboard chip, or whether there was some caveat that made this worse than just running everything off the dedicated card.

[Haldi]

AFAIK you can only use the onboard GPU as additional Monitor.
You cannot use AMD Eyefinity or nVidia Surround with the Onboard GPU.
Also if you start a Game on the additional Monitor it will use the iGPU and not the dedicated.