Originally Posted by
FoxtrotOscar
I believe if one leg is a BA flight number then the whole booking should appear in your BA app, which depending on class/status may give you better access to select seats. Am not familiar with AA's app so can't comment if it's advantageous.
You would be able to select seats on the BA operated flights (even if on the AA code) via the BA app/website but for any AA operated flights (even if booked the BA code) you would be sent to AA to select seats.