I'm not sure about that. I booked AA-IB-IB and IB-IB-AA trips from LAX to LIS and back recently, and AA seemed to have "married" connectors from wherever IB had availability (on the east coast) to me at LAX. I got LAX-JFK in U and BOS-LAX in U that I don't think I could have gotten on its own (but then I can't be sure because I didn't try before i booked, and if it was U1 then it would have disappeared once I booked). Now, these were both Saturday flights, so that may have helped availability in U in general, I dunno.