Use dietpi-drive_manager to add the drives and assign perm UUID mounting.
DietPi-Sync is one option, good for daily backups, front end for rsync:
If your master drive is /mnt/usb_1, you would setup
/mnt/usb_1 = source
/mnt/usb_2 = target
Enable delete mode if you want a exact mirror of the master drive.