The only way to guarantee that this occurs would be to create partitions in your library, and then create separate jobs for each night, pointing to one of the partitions. Use an OPP (Overwrite Protection Period) of six days to reuse the tape on the same day each week for the media set the tapes belong to
See http://www.symantec.com/docs/TECH27909
Be aware that if, for whatever reason, the tape in the specified partition is not overwriteable, then the job will fail. BackupExec will not look in any other partitions for alternate media
