I don't know the answer. On Intel-based boards you can usually only install one RAID on the PCH-controlled SATA ports (plus a second RAID on added ASMedia-controlled SATA ports). The desktop chipsets tend to support limited flexibility in RAID configurations.
Maybe AMD X370 is different (better).
Why do you want to install a pair of mirrored RAIDs?
2-drive RAID1 and 2-drive RAID1, functions as two logical drives with mirroring
vs
4-drive RAID10, functions as one logical drive with mirroring, plus striped capacity and performance of two physical drives
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[/Korth]