Right. You can't use 2 coupons for 1 granola bar. But you can use 2 coupons when you have, say a granola bar and an eraser. Or 1 granola bar and 1 item you had a raincheck for. The system does allow it, it's just that most of the cashiers don't know how to do it.
This is another good site for info about Walgreens:
i heart wags
There is a similar site for CVS:
i heart cvs