I finally resolved this issue and the cause was something I never would 
have suspected.  It turned out that my tape drives were flagged as 
"needs cleaning", which apparently causes NBU to unload a tape 
immediately after a stream is written and ignore any type of normal 
unmount waiting period.  I reset the mount time on my drives to clear 
the flag, and suddenly the same backup job used one tape.

I need to give credit to the folks on the Symantec forum for the solution:

https://forums.symantec.com/rss/message?board.id=21&message.id=41109

Bryan

Bryan S. Leaman wrote:
> I am running an RMAN backup to a storage unit with multiple drives on 
> 6.5.1.  The RMAN backup is allocating only one channel as it's a small 
> database and there is no need to use multiple drives.  However I'm 
> noticing that for each new RMAN backup piece, NBU mounts a different 
> tape in another drive instead of using the one that is already loaded 
> which was used for the previous piece.  So it ends up alternating back 
> and forth between several tapes and using many drives in the process.  
> This doesn't seem very efficient.  Is there any way to prevent this?  
> I don't remember seeing this with 3.4 from which I am in the process 
> of upgrading.
>
> Example: RMAN backup with one channel, and 3 backup pieces
>
> Stream/piece 1: tape 1 - drive 3
> Stream/piece 2: tape 2 - drive 2
> Stream/piece 3: tape 1 - drive 1
>
> This behavior also seems to be the same that some of us have seen 
> recently with catalog backups, in which it will create several 
> streams/jobs and put them on different tapes for one catalog backup 
> (thread "Catalog backup = 4 separate jobs???").
>
> Thanks for any help on this.
>
> Bryan
>
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