Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Keep in mind that BA and United each offer one-way awards, so you might be able to fly out on UA and back on AA, if you can find the seats.
Also, certain connections on AA would only boost the number of Avios required from 12,500 to 17,000 for a Coach MileSAAver one-way. Examples: JFK-DCA-LAX; JFK-BOS-LAX; JFK-SFO-LAX; JFK-SAN-LAX; JFK-LAS-LAX; JFK-RDU-LAX; and LGA-RDU-LAX.