Backup mybook live duo to 2 USB drives

 have two WD USB drfives, exactly the same, formatted wit HFS+

I want to rotate them using them for my safepoint backups.

What’s the best way to do this?

Just pick a rotation (monthly?) and swap the drives. How will the MBLD handle this? 

There will be two separate backups

the MBLD will backup the drive that is currently connected so pretty much it will just backup the same data twice