I tried everything, but was unable to get bifurcation working on the X9DRL-iF with Supermicro AOC-SLG3-2M2 cards inserted into the X8 slots and set to X4X4 in the BIOS. Regardless of booting legacy of UEFI, the bifurcation setting did not do anything in BIOS version 3.3, Linux only saw a single M.2 SSD in the slots.
OTOH I had complete success with bifurcation on the X9DRi-LN4F with the same Supermicro cards, but it required downloading BIOS version 3.4. Unfortunately, due to it being an EOL product, Supermicro said they are not going to release a fixed BIOS for the X9DRL-iF, even though it would likely be an easy fix.
A not too expensive solution would be to use an Asmedia ASM2824 based adapter card instead of the AOC, these are available for around 150-200 USD/EUR and provice the same functionality without the need for bifurcation support on the motherboard:
Lycom DT-130
Ableconn PEXM2-130
Ableconn Technologies
ableconn.com