@redeuxx Welcome to the Rockstor community. Sorry but this one is a little more involved than it is with Rockstor’s data disks as there are as yet still unsolved / outstanding issues upstream with grub and grubby to make this a straight forward process, and also some issues with btrfs defaulting to not mounting degraded pools by default; which would of course block system booting in this event.
There is however a rather convoluted but proven method to install Rockstor on to an mdraid mirrored setup (ie RAID1) over in the Official Docs and more specifically the Mirroring Rockstor OS using Linux Raid howto however this is a long winded process and should only really be attempted if you are happy with manual mdraid repair techniques. But the result is a redundant system disk arrangement that in the event of one of these system drives going bad should boot anyway using the remaining mirror member and if Email Notifications are enabled you should receive an email with critical mdraid event info.
This will however require Reinstalling Rockstor and if you want to preserve your current configuration then the Configuration Backup and Restore document may be of help.
Hope that helps.