I've just made a couple of reservations stacking both. I use Safari on a Mac. I go to AARP and login, then go to the travel section to get the link to go to BA. This takes me to the BA web site. When I pull up an itinerary there I get the "Discounted" fare in a block under the actual fare. I'm sure you have been this far. At this point I usually go to the point where I put in the discount code (Booking Summary page) and log in to my BA account. Then when I put in CHASEBA10 the pop up sometimes stays up, but just now with a trial booking it went straight to show a discount applied.
Good luck