If you have an Overwrite job and you want to immediately follow that with an Append job to the same tape, both jobs must use the same Media Set, otherwise the second job will grab a tape from a different set (or a new Scratch tape), rather than the tape from the previous job. Limit this Media Set to only these two jobs, and create separate Sets for other jobs.
Set the Append level on this Media Set to a short duration - one day or less, but longer than it takes the previous job to complete. This assures the append job does not go to an older tape in the Set.
Then overlap the time window for the jobs. For instance, if the first starts at 9:00 and takes a minimum of 30 minutes, set the second to start at 9:15. Then you are sure it will immediately follow the first job.
Hope this helps