With your assertion that you have (by implication, only) two options. To me where I got the OWE ticketed is irrelevant. As regular readers will know I almost always get the ticket open-dated and then simply call the reservations line of the airline I wish to fly on and make a reservation for the sector I wish to fly, quoting the ticket number when I do so. I guess that my average AONE3 has 4 or 5 PNRs (reservations) containing sectors from it during its life. It also probably changes ticket number and ticket stock at least once during its life (normally from the Emeco-issued AA stock to a CX@HKG-issued CX stock).

But I also have situations where some of those PNRs also contains segment which are ticketed on an entirely separate ticket. This is not a problem either.

Moreover, it doesn't seem to matter if, when making a reservation, the ticket number quoted doesn't actually contain the sector which I am reserving. This is often the case in advance of a re-route, where I will make the reservation in advance, but only get the ticket reissued to match the reservation when I arrive at the airport to take the flight.