Hey, jb1 - I saw your post on ASRock forum about the bifurcation on Z170 Gaming-ITX. Did you manage to get a direct answer from them? As I see that the thread wasn't answered...
Nope, they never replied. I just bumped the thread over there in hopes that they get back about it.
From the manual for this board it doesn't seem like there are any options for bifurcation, which seems to suggest that the BIOS at publication didn't support it. However, I remember seeing something that suggested that they would add support for 100 series chipsets.
I ended up going the X99 route!