Some stores regularly have BOGOF items. Publix has created an entire marketing plan with that in mind -- to save their customers money. However, for those who aren't "in the know" (like you are now), they can sometimes be less cost effective than they first appear.
One of the things you need to know is how your BOGOF item is going to ring up at the store. Some stores ring them up at half price, so that you can buy just one at half price (and if you only have one coupon, just buy one item -- you don't have to buy two). Other stores ring them up as the first item at full price and the second item is free. This is the scenario we coupon folks like least. You typically see this at drug stores.
If you have a BOGOF item that rings at half price you can use TWO coupons for that item. For example, tonight at Publix, the Betty Crocker fruit roll-ups were BOGOF at $2.79. Because I know that Publix rings each item at half price, I was able to use two $.40 coupons (which were doubled) to make each box $.60 [$2.79/2 - $.80 (dbl coupon)].
The converse is true if the item only rings up the first item at full price, you can only use one coupon because technically you are only purchasing one item.
If you have doubts, ask a store manager (and often clerks know this as well). They are usually very helpful.