I'm wondering what backup mode the source backup jobs are using. Forward or reversed incremental?
I'm asking since in case of forward incremental mode, the tape job should proceed without any issues. Meanwhile, in case of reversed incremental, it should be interrupted. May be switching mode would be an applicable option?
Otherwise, you can, probably, put into use custom PS script that will check the state of backup to tape job, and, start backup job only if the tape one isn't running at the moment.