Maybe all replies went direct but may also have no answer, so have one.
Quote from TSM for AIX MgSysSAN Storage Agent User's Guide (v4.2, p.18):
"Map all drives. Problems can occur if you do not define drive mappings on 
the server for each drive in a library. For example, during backup 
operations, all drives that have been mapped can be used by storage agents 
backing up data. Backup operations will fail if you try to use other 
drives in a library that have not been mapped."
So this is expected and documented behavior. Have in mind that agent is 
just a server with partially disabled functionality (and lower price).

You have two options:
use SAN Data Gateway. Benefits - all drives are SAN-attached thus 
shareable. Negatives: additional investment (management decision, time, 
money, etc.)
define two libraries, one over SCSI drives and second over FC drives. 
3584 allows you to have more than one library control path. Two 
suboptions: