I always believed this to be the case also, but I just had a AAA
travel agent tell me I could put in my 40% code and they could give that to me ON TOP OF the AAA discount! He also said I would have to call DIsney and see what's available with the pin code discount, then call AAA back to put in my reservation. That seemed weird to me right there.
I was skeptical, so I called Disney reservations to inquire - the guy I spoke to said, "Well, we're not familiar with AAA's booking procedures, so you'd have to talk to them, but I imagine if they wanted to give you their discount on top of the 40%, they could."
So here's me - !!!????
I currently have a pin code package booked with a (non-AAA) TA, who I'd prefer not to take the business away from if this is a load of hooey, which I still suspect it might be. Maybe I'll call AAA again and see if someone else confirms this......