American Airlines and a split order
Lots of large (and not so large) airlines operate similar sized machines from different manufacturers -Air Berlin, SAS, THY, Lufthansa, BA (A320 family plus B737), US Air, Even beofre the amalgamation with NW Delta flew B737-200 and MD80's. Recently Southwest has hinted at a 2 model fleet. American flew A300's and similar sized Beoings for years.
For the size of AA, split fleets at different bases means not a lot of economy of scale is lost.