NetBackup 7.5.0.6 SLP - Primary Copy Problem

I'm having a problem setting a primary copy with SLP.  I suspect what I want to do and what I can do are two different things.  Here is what I have.

• 1x Master/Media server - NBU 7.5.0.6 (Oracle Linux)
• Library A: 6x LTO4 drives.  Tapes from here go offsite.  Most jobs go directly here.
• Library B: 1x LTO5 drive.  Want to make this purely for duplication with some manual archive jobs.
• 4TB basic disk (C?).  Any two of my largest backup jobs fill this up.  Currently used to allow small jobs to run while tape drives are in use.  Restore requests are rare for these.

I would want to write to A, and duplicate to B, leaving B as primary copy for restores.  B only having one drive seems to only be useful as a final destination as I cannot write new backups while duplicating to A.

Any ideas?