The Intel spec for Gen 8 Chipsets (this board should have a C226) shows the "server" chipsets (C222,C224, & C226), plus Z87, have support for bifurcation of the PCIE Express Graphics (PEG) slot (the x16 slot). Supported configurations are 1x16, 2x8, 1x8, or 2x4. For the Consumer/Business chipsets this slot is fixed at 1x16. See page 56:
https://www.intel.com/content/dam/w...datasheets/8-series-chipset-pch-datasheet.pdf
I believe that the motherboard may already have this slot fixed at x8 (to have enough lanes for a second slot to be x8 electrically?). So, presumably, splittng it to 2x4 would be the easiest option, assuming it can be done.
As
@techtoys menitions, knowing if its supported in the hardware itself is the first step, I think you're good there, if you stick to the PEG slot. Unlike the C6xx chipsets that support bifurcation of multiple slots, the C22x only support bifuration of the one slot.
As for a guide on modifying the BIOS to enable bifuration: I've not seen anything specific to enabling bifurcation on the C22x chipsets, but I've seen this guide on the Win-Raid forums, which is focused more on X99/C6xx, but may be of help.
BIOS Modding Guides and Problems » [Guide] - How to Bifurcate a PCI-E slot
There's an NVME boot BIOS mod thread on the forums here that also mentions bifurcation I believe. Its for the prior generation chipset, but has some good information on BIOS mods in general:
https://forums.servethehome.com/index.php?threads/nvme-boot-with-supermicro-x9da7-x9dri-f.13245/
There's a Win-Raid thread mentioned in the post above that, while its focused on BIOS mods for NVME booting, is very detailed on the BIOS mod process and may be useful in understanding the process as a whole:
[Guide] How to get full NVMe support for all Systems with an AMI UEFI BIOS