Start by reviewing this document on Symantec Tech Support:
http://seer.entsupport.symantec.com/docs/284575.htm
It'll point you in the right direction by using "duplicate backup sets" to move the disk-based backups off to tape.
The Duplicate Backup Set template enables you to use a multi-stage backup strategy for backing up data to disk and then copying it to tape. The duplicate backup template does not replace the existing Duplicate Backup Sets option. Instead, it provides an automated, alternate method of duplicating backup sets. It allows for multiple levels of data duplication either within the backup window or outside of the backup window.
