You want one of these. They have a PLX switch internally so you don't need bifurcation.

This one works perfect in the cMP5,1:

High Point SSD7101A-1 NVMe RAID Controller https://www.amazon.com/dp/B073W71K4Z/

Make sure you put it in an x16 slot so each drive gets all four pcie lanes it needs. NVMe drives each use 4 lanes so you can run 4 drives in each x16 slot without incurring a performance reduction.