In BrightStor ARCserve there are a few options to have a copy of a tape for offsite storage.
First off the current vesion of ARCserve Backup has staging to automate backing up to one target and then copying it to another.
In your version you can setup two tape drives in the same Device Group and then use the Tape RAID Option configure for RAID 1 mirroring so the data will be written to two tapes at the same time.
Another option is tapecopy.exe which is a command line utility that comes with ARCserve. It can be used to copy the data on one tape to another tape in a second drive.
Don’t have a second tape drive, no problem. Setup a FSD File System Device. Then backup to the FSD and use tapecopy to copy the data from the FSD to tape. Run the tapecopy job twice once for the local copy and once for the offsite copy.