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So my takeaway...

The M.2 connectors support M.2 SATA SSDs or M.2 PCIe SSDs and support RAID configuration.

Please note that an M.2 PCIe SSD cannot be used to create a RAID set either with an M.2 SATA SSD or a SATA hard drive. Refer to Chapter 3, "Configuring a RAID Set," for instructions on configuring a RAID array.

You can't Mix and Match PCIe and SATA  - RAIDS are independent ;) 

But you can Mirror the 970 and 980 (keep in mind one is 500 and 512 and there are speed differences)

So, according to Chapter 3:

If you want to use NVMe PCIe SSDs to configure RAID, make sure to set NVMe RAID mode to Enabled

After that it doesn't really say if you can create an "initial" independent drive and later mirror it, But I would say the larger picture here is getting the Windows drivers correct.

When Windows installs and analyzes the HD controller and decides on drivers it doesn't like them changing after the fact, especially on boot devices.

Typically, you always put the Controller into RAID mode even if your not defining an array so the Windows installer knows to use the RAID Drivers and not the NVME or AHCI SATA ones.

After you should be able to add \ removed drives within the BIOS (or GUI tools if installed)

Just Remember your settings and don't have a panic attack when you flash or default your BIOS and it controller reverts back to AHCI mode and windows does'nt boot  ;)

Good Luck, 

Mike