I don't believe the Asus Hyper M.2 card exposes each NVME drive individually (when the BIOS is set to RAID mode) - could be wrong.
If you want to set your secondary PCIEx16 slot to 8x in the BIOS, then you can use one of those ABLECONN PEXM2-130 Dual PCIE NVME Adapter cards. It has a ASMEDIA ASM2824 Chip in it (PCIE Switch) which will allow you to get full bandwidth from 2 - X4 PCIE 3.0 NVME Drives. Exposes each drive as a distinct unit.
Not cheap though.
You can probably set one up as an OS drive, and the other as a DATA drive - probably need to mod your BIOS and inject the NVME module though so you can boot from it. If you want max throughput (let's say 8K video editting), use Windows, have regular backups - I'd say combine both NVME drives as RAID 0 using Windows 10 storage spaces.