It's not a backup, it's a mirror, meaning everything is stored on both drives at the same time and any change is written to both drives, there is never a delta between both drives.
Meaning there is not a first/second drive, it's on both and any one drive can die (or unplug for that matter) and accept Truenas shouting vary angrily at you that 1 drive is missing, everything is still working as if nothing ever happened.
You don't even need to do anything at this point, Truenas is handling everything automagically. Once you got a new replacement drive, you put it in, setup the mirror in Truenas/Hexos again and you got full redundancy again.