Originally Posted by corporate-wage-slave

You need the AA PNRs and it may be you have 2 of them. That's different to the BA PNRs, but you may find one of them ion the screen of BA's seating menu. Otherwise contact AA to look up the AA PNR and then use the AA App or website to assign seats. This will only work if you are ticketed, and if you have changed your booking from the online version then ticketing may not be complete yet. BA and AA are on different GDS systems, hence the problem and hence the different PNRs.