I am doing the same sort of thing. i run the backup to disks during the night, then to tape in the morning. After the B2D's are copied to tape each morning, the B2D's are overwritten with each nightly backup. I have used policies to do these, because otherwise you will find that there is no way to restore the B2D that has been written to tape. You will need to do the following (assuming you already have the B2D folder created)
1. Create the job selection lists
2. Create a New policy, select new Backup template, and create your B2D job using the selection list you created in step 1
3. In the same Policy Create another new template as a 'Duplicate Backup Sets template' which will copy the B2D to tape.
4. Add a new template Rule as 'After completes, start , then select the B2D as first template, and B2D to tape as second template.
I have been running these backups for a few weeks now with no problems at all. When you come to restore the data from tape, you will be able to see the complete folder structure as you can on a normal tape backup.
Hope this helps.