A couple of years ago I flew SFO-LAX-SAT, first segment on AS, second on AA, and back. Purchased on AS.com. Outbound I was able to check-in online on AS.com and received both boarding passes. On the return I could not check in on AS.com but was able to check in on AA.com. (I got the AA PNR from AS CS) But, I could only get the first (AA) segment. WE had some delay and I barely made the connection. AND I was not checked in! (I had somehow figured that.) I'm glad they hadn't given my U seat away. When I approached the desk and asked for a BP, they immediately asked if I were connecting from AA!