I have never been able to use 2 coupons on National but I do change the contract ID and generally the Emerald Club ID gives the best discount.
AAA also gives a good rate but many times when you go the end page of the reservation process the best final rate is with the Emerald Club. I'm a little nuts (like many here) and check my rates daily. On my last trip it started in the $300 range for 12 days for an Intermediate....final price was $200. Also depending on how soon you are going the Entertainment book took a couple of weeks to arrive.