I experienced something similar with a Rosewill R2S-B dual-bay enclosure. In JBOD mode, with one drive the Live Plus recognizes it fine but with two drives (in my case a 1.5TB and a 1TB drive) only one is recognized and accessed.
On a whim, I set the unit up for BIG/SPAN mode with both drives installed. No joy, but I didn’t realize the drive had to be reinitialized and partitioned from scratch in this mode. Connected the unit to my Win7 computer and used Computer Manager to set it up and format it as one big partition.
After copying a bit over 2TB of movies to it and connecting it to the Live Plus again, success! All of the files are accessible and can be played from the local drive. So, it appears that if a dual-bay USB enclosure supports BIG/SPAN mode you should be able to make it work with two drives installed.
The only frustration I have is that when browsing from the local drive the cover art only shows up when navigating by folders, and only for the folders themselves (not for the movie files, even when a .jpg in the folder has the same name). And Selecting the All Video option just shows a bunch of generic icons, period. This happens even after power-cycling the unit and manually browsing to all folders.
Even so… I’m happy to find out that my dual-bay drive enclosure isn’t completely useless.