Firstly one tape would never be best practice as if the tape develops a fault all of your backups are gone.
Also if you overwrite the tape on every use (or even when it fills then if something happens to fail the job that actually does the overwrite you will have a period of time with no backup at all - best practice is do not overwrite your backups until you have a new sucessful backup on a different piece of media/tape
Add to that the fact that once the tape fills up it will need a second tape as you can't continue the job on the beginning of the same tape (as an overwrite is a complete destroy of all backusp on the tape)
Also be aware that Backup Exec will move overwriteable tapes between media sets if it needs that tape to continue a job configured for another media set, a media set is not a physical barrier to another job using the tape, the only ways to achieve physical barriers are:
1) if using stand alone tape drives don't insert the wrong tape
2) if using a library partition the slots
Note both of the above can cause media insert requests and jobs either failed or waiting for input overnight. and if the tape is not overwriteable it cannot be moved between media sets as when a move occurs it does an overwrite.
