It depends on which operating mode you choose.
You can configure the hard disks as individual hard disks, in which case they appear with two drive letters. You can read each one individually. However, you can connect both hard disks as Raid1, then the whole thing appears as if you only had one hard disk, but in the background the two disks are duplicated, so that if one disk fails, the data is still available on one of the two hard disks. This mode of operation only protects against the technical failure of one of the two hard disks and does not replace the backup. However, in the event of such a technical failure and the replacement of the defective hard drive, one can resume work much sooner, because restoring a backup of 12 TB can take about a week, even with USB 3. However, My Book will have to deal with itself until the complete data migration to the replaced hard disk and will react more slowly. Since the Raid controller is inside the My Book, a native Linux could be working here and it might not be possible to read out the individual hard drives, e.g. in a docking station. However, I have never tested this because the total failure of the device is much less likely than the failure of a hard disk and I have set up automatic backups.
With the last possibility of making a large data room out of the two hard disks, it is not possible to read out a single disk with only two hard disks because the data is distributed randomly over the two drives.