You can use either on or two B2D folders.
Personally I'd use one, and keep the fulls and Incrs together.
Use two media sets,
Full, Overwrite, OPP 13 days, append 0
INCR Appen else Overwrite, OPP 8 days, Append 6 days
Append is figured from when the first backup set on the media is opened
Overwrite is figured from when the last backupset on the media is closed.
One question I have is
What are you going to do for offsite backups? Say your building burns down
Will yoou have two External HDs? if so, remember that you must mount each drive on the same unique drive letter each time (that is drive1 as S: and drive2 as T:)