My current system is a ASUS ROG Maximus Hero Xiii and I am using WD Black SN850 NVME's in the M.2_1 and M.2_2 slots. The 1st PCIe slot contains a RTX-2070 and all other PCIe slots are free.
If I read and understand the manual, I will get faster speeds out of the M.2_1 drive slot than the M.2_2...