Just to clarify, and ensure I understand correctly, this will work when your motherboard supports PCIe bifurcation?Au contraire! ... Did you expect to find postings like "I've got this mobo and the bios has this option and it works."? Like Jell-o, it's hard to screw up bifurcation. It's dead simple.
I believe you've "thrown the baby (ie, bifurcation) out with the bathwater (the AOC-SLG3-2E4R)". Something on that redriver card is not playing nice with the X9DRI-F.
Consider canceling the 2E4 (@$150-200?) and getting instead either:
1. PCIe x8 to 2x SFF-8643 adapter (w/1 cable) [Link] for $45.
or 2. PCIe x16 to 4x SFF-8643 adapter [Link] for $35.
I understand, and appreciate, your hesitancy to buy from unknown/distant sellers. I am quite risk-averse myself. But, after seeing that others (on STH) had purchased from each of the above sellers, I also did, with no regrets. (In both cases, delivery was <17 days after order.)
I'm not cheap -- I'm capital-efficient.