Originally Posted by skunker

AA started to do the double PNR for codeshares starting a couple years ago. If you book on AA.com and have a codeshare on a partner (I know TATL JV specifically does this, don't know about other codeshares) then there will be two PNRs in your account: 1) the full PNR will all flights as booked, including codeshares, 2) a seperate PNR that includes the codeshare flights as AA primes. The second PNR allows you to select seats, but doesn't really do anything else.

Here's an example from my trips for a AMS-HEL-LHR-JFK-LAX flight with the LHR-JFK a BA code on AA metal.


I have found that the best way to select seats on TATL codeshares is to use rj.com. For some reason their site usually cuts through all the codeshare BS and it allows you to select seats, or at least view what you have selected.
For IB/BA/AY PNRs you can also use checkmytrip.com for an overview of the PNR