AM4 nvme RAID is a nightmare to set up, but I will try and give you the fast version, the rest you will have to figure for yourself.
download: AMD RAID Driver (SATA and NVMe RAID) Revision Number 126.96.36.199 File Size 711 KB Release Date 6/7/2018 and extract the files to a USB stick.
First unplug all other drives in your computer, just leaving the 2 nvme drives you want to raid together, it will make it easier later on.
Now boot to the bios, disable CSM support, change SATA from ACHI to RAID, now reboot back to the bios.
You will now have a new sub menu called Raid Xpert in there choose the RAID version you want, ie RAID0, RAID1, RAID10 etc, choose the drives you want to use, select both your nvme drives, be aware that as soon as you apply the settings it will wipe everything on the drives so you will need a copy of Windows on USB or DVD to install.
Now comes the confusing part, once you have finished configuring your RAID setup, reboot and boot off the USB/DVD containing windows, go through the setup until your reach the point where it asks you which drive you want to install too, you can probably see both of your nvme drivers in the list, DO NOT DELETE ANY PARTITIONS THEY CONTAIN VALUABLE DATA, at this point, if its not already plugged in, plug in the USB containing the AMD drivers you downloaded earlier, now select load driver, navigate to the usb stick containing the AMD drivers, there is 3 to load, first one to load is RC Bottom, when that's loaded you may see your drives disappear, select load driver again, goto the USB stick containing the AMD drivers and select RC Storport, when done finally load driver again, goto the USB stick containing the AMD drivers and load RC Config, you should now see one drive available, double the size if you chose RAID0, now install windows.
When you have finished installing windows, shut down the computer and plug in your other drives, these will appear in the bios as NON-Raid drives.