As far as the firmware is concerned the My Book Live Duo is essentially a My Book Live with the addition of a second drive and some Raid arrays.
The disk uses a GPT partition table. Below is the script that is used to create it.
mklabel gpt mkpart primary 528M 2576M mkpart primary 2576M 4624M mkpart primary 16M 528M mkpart primary 4624M -1M
Partition 1 is ~2GB and holds a copy of the OS
Partition 2 is ~2GB and also holds a copy of the OS.
Partition 3 is a 500MB swap partition
Partition 4 is the main data partition. The -1M in the mkpart command is an instruction to use what ever space is left on the disk.
The duplicate partition for the OS is obviously a case of resilience for the My Book Live. I assume on that system they are set up as a single Raid array? It's not ideal as both partitions are on the same drive, but it does mean that a disk error in either partition doesn't stop the system running and does give the user to time to safeguard their data.
I guess that in the case of the My Book Live Duo, to avoid undo complexity in the shared source code, Western Digital decided not to make any changes here. It does mean that there is now complete overkill as there are now four copies of the OS partition - two on each drive.
In setting up the raid arrays we have:
/dev/md0: comprising /dev/sda1, /dev/sdb1: OS
/dev/md1: comprising /dev/sda2, /dev/sdb2: OS (alternative copy)
/dev/md2: comprising /dev/sda3, /dev/sdb3: Swap partition
/dev/md3: comprising /dev/sda4, /dev/sdb4: Data Volume.