In light of the downgrade in the OW policy on interlining for separate tickets, I need to learn a bit more about the process for adding multiple bookings (ticket numbers) on a single PNR. With the OW changes, it seem multiple ticket numbers on a single PNR is still protected. As such:

1) How can multiple bookings (ticket numbers) be added to a single PNR?

2) Who is responsible or able to do this?

3) Are travel agents or those with access to a GDS the only ones who are able to do this? Or can you buy two tickets directly from the airline/s and get the numbers to "sit" in the same PNR?