Can someone give me brief overview of how to backup to disk then tape?

Its actually fairly simple with 11d. What we do and has worked for us is to create your backup-to-disk folders under the media options. Create a backup job as you normally would for a tape device but in the destination location of the backup job properties select to send the backup job to the disk location you have configured. Once this has been completed make a ' new duplicate backups sets job' configure this job to point to your existing tape drive and select the backup to disk job as the backup to duplicate. This will then duplicate your backup to disk job to tape once it has completed.

Needless to say there is more finer detailed configuration required on a per job basis such as selections, credentials, aof options, etc. But this should get you pointed in the right direction.

Hope this helps.

Is there a step by step guide available anywhere on the web that I can follow? This would help greatly.

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.