I think you need to look at the staging backups option which is available within v10, as that will then allow you to backup your data to your NAS (I'm assuming this is where you backup to disk folder is located, as apposed to it being the actual source of the data), and then from there back it up to your library.
I haven't used it myself, but from the presentation I saw on it last year (admittedly for Net Backup, not Backup Exec), I believe as part of that you can configure how far back the disk copy goes, thus enabling you to keep multiple days backup on disk, and rotating it accordingly.
As far as pointing the backup to the library, you should be able to simply point it to the library, though from the fact that you are seeing two devices listed I wonder if you have it configured correctly. On our Powervault LTO library we only have one entry for Dell1, and no Dell2 listed. Within Dell1 you should see two entries, one for the drive itself, and the other for the library. By pointing the backup at Dell1, it should then make use of which ever tapes are available in the library for use. That is unless you have partitioned the library, in which case you will see entries like Dell1 , as options for the backup device in the job setup.
One other thing I would comment on. If you are only planning to move the Sunday tape to the bank, I would suggest getting a fire proof safe at the office, and storing the other tapes in there once used, otherwise if something like a fire happens at the end of the week you've defeated the object of having tape backups for disaster recovery.