Hi Jeromy,
You should have 2 media sets, rather than (or as well as) 2 B2D folders.
One media set is for your diffs, the other is for your weekly fulls.
For your weekly fulls, you want an Overwrite Protection Period of 7-13 days (i.e. don't overwrite a full backup for that time, during which another full should take place).
For your daily diffs, an OPP of 6 or 7 days should do (i.e. you want to keep the diffs around until a new full has been done)
The changes above also take into account that you should have 2 fulls lying around. If you have one full backup, and you start overwriting it and something goes wrong, then you are without a full backup. By having two fulls, if something goes wrong then you still have the previous full, and the differentials will continue to make the backup set up-to-date.
Also, if you are backing up to disk, you may as well do incrementals rather than differentials. It'll save disk space, and your restores will be hardly any slower.
See http://www.backupexecfaq.com/faq/concepts/understanding-media-sets.html to get an idea of how media sets, append and overwrite protection periods work.
Hope this helps
---cheers
shirmal