Thankfully Walgreens has posted the official coupon policy on their website!
A few things I'd say are big...>
::When items are featured in a Buy One, Get One Free promotion, up to two coupons can be used against the items being purchased, as long as the net price does not go below zero for the items being purchased.
::When purchasing a single item, Walgreens accepts one manufacturer coupon and applicable Walgreens coupon(s) for the purchase of a single item, unless prohibited by either coupon offer.
::The coupon amount must be reduced if it exceeds the value of the item after other discounts or coupons are applied. (For example, a $5.00 coupon for a $4.99 item will result in a $4.99 coupon value).
I'd print this out and take it with you when you shop! (I've already done so!) And if your store is saying they can’t reduce a coupon or accept two coupons on a Buy One, Get One Free sale or that they can’t accept a manufacturer’s coupon and a Walgreens coupon, now you have a written corporate policy to show your cashier or manager!