Want to take a copy of back up off site

We have about 3 TB of data across 4 virtual servers running Redhat Linux on an IBM Blade Center. Net Backup for back up on IBM LTO Autoloader.  Compliance requires 15 years of Data to be available.

Q 1.  I would like to take a full back up out at least once a year.  Do I need additional tape drive to do that or the single drive of the autoloader help me create a copy that can be taken off site?