When looking into these boards for a new build, I noticed that the manual of the ACE said on p64 that it has a BIOS setting under Advanced called PCIe SlotX Lanes configuration.
This is for the MSI M.2 Expander series cards, which add M.2 options but need bifurcation. So I am assuming this setting is actually about PCIe bifurcation.
EDIT: after a lot of googling I found a blurry hand cam filmed overview of one of the latest ACE BIOSes and it seems both PCIe Slot E01 and E03 are supported. And it is about bifurcation - x4x4x4x4 / x4x4 depending on the slot. So that answers that part of my question.
However, I still need confirmation the Unify also has these settings.
Also, since the Unify is based on the ACE (just minus a few trinkets), can any Unify owner also please check if that option is in its BIOS and what the options are? The Unify manual is very limited on BIOS info.
EDIT: thanks to an owner chiming in privately, I now know the Unify does have the bifurcation support on the second x16 slot.
Topic closed.
Thanks! Keep safe everyone.