Well expanding on this topic.

Last week I booked two tickets online. One ticket was my ticket flying SHA-oHKG-PVG; the other ticket was for my partner SHA-oHKG-SIN while logged out of my account. The outbound is the same flight whereas my ticket is technically a roundtrip the other a one-way with a stopover.

For my partner's ticket I booked with her AA FFP number (no MPO/AM account) and I added my own MPO number to my own ticket. The only thing that I did was pay for both tickets over the phone with the same credit card by calling on separate occasions, and to my surprise, I just tried to check-in into my own flight and it showed both passengers on my booking even though at no point did I indicate that the two PNRs were to be linked. In fact these two tickets are booked on separate ticketing carriers, mine on KA 043 and my partner's ticket on CX 160 ticket.

So again I suspect the system is smart enough to see that both bookings are flying the same first sector and paid with the same credit card. I am able to choose otherwise "extra legroom" seats for my partner without any issues, so there is definitely some sort of linking going on in the background. In following the MPO-CC linking idea, if I enter the ticket with the other ticket (not MPO/AM) with the ticket number, I cannot see my own booking.