AA is the operating carrier xSFO and AA's rules regarding checked luggage as to # of pieces, weight, time deadlines and connections, all apply. OP would get the same answer if his onward flight were on AA itself or any other carrier which interlines with AA.

Might an agent override the system and do this for a passenger? Of course. But, the passenger can't count on this and there is no way to determine in advance whether it will work.

This is true whether you are on the same ticket (bookings and PNR's are not relevant here) or multiples.