If you have no drives, you have no OS. It’s impossible to even boot a duo without at least one disk that already has firmware installed and partitioned.
Tony is correct. If you have some skills you can download some MyBookLive Duo images from the bottom of this page http://mybookworld.wikidot.com/backup-images-of-mybook
There are also some instructions on how to get the images on the drives but you need to know what your doing
Greens and Reds will work in the enclosure, not sure about any other drives
Don’t own a Duo myself so I would be of no help beyond this point
buy a new WD mybooklive duo, take out one hard drive, put it in this empty one then put two new ones in the two gaps? Then restore the second drives in both units?