OK, Advanced Mode then Advanced -> System Agent Configuration (I think). There should be a Graphics Configuration option in there somewhere and you want to set "iGPU Multi-Monitor" to Enabled.
It's actually the exact same on my P8Z68-V LE. I just forget exactly what part of the Advanced section it's in, since I've only ever enabled it once to try out Virtu, but I just disabled it right after and forgot about it, lol, and the manual doesn't exactly make it very clear.