1 of 2 ways:
1. If you have a library with 2 drives (or even 2 stand-alone drives), then run a Duplicate job which can be setup easily. Simply duplicate the data from tape 1 to tape 2, and then you can erase the tape.
2. If you don't have the hardware in #1, then you need to restore the data to disk. Once done, put in the 2nd tape and then back it up before erasing the tape.