You need to mod the current bios, here’s all the guides you need:
https://winraid.level1techs.com/t/guide-adding-bifurcation-support-to-asus-x79-uefi-bios
https://winraid.level1techs.com/t/guide-how-to-bifurcate-a-pci-e-slot
https://winraid.level1techs.com/t/guide-pcie-bifurcation-mapping-iio-iou-to-pcie-slot