I'm not following your verbiage possible (what do you mean by normal tape job cycle, or source tape job?)
As to source job, I've made a typo and written source tape job, instead of source backup job; apologize for confusion.
Normal tape job cycle means a cycle during which a backup to tape job is just copying files, and not creating virtual synthesized full backup directly on tapes. So, the next question during Saturday's cycle you're willing to create a virtual synthesized full backup on tape or just make backup to tape job copy files?
Currently if the tape job starts Saturday morning 9:00am (synthesized full on Friday) and my daily jobs start on Saturday night, it will break the currently running tape job.