As you have partitioned your library into 1 slot partitions and have a job that fills the tape in slot 5 and goes to two tapes then I would suggest you make the partion itself contain 2 slots instead of one and then (as long as the media is not overwrite protected) the job will use the tape in slot 6 when the one in slot 5 is full.
Notes:
- You cannot make jobs flow from one partition to another
- You cannot create partitions containing slots that are not adjacent to one another
