I wonder whether this is possible: I buy a ticket on airline A for me, then I buy a ticket on airline B for my kid, same flights, and airline B puts my reservation information into the kid's record such that the kid is not marked as an unaccompanied minor. Is this possible and allowed, or does software inflexibility or airline rules prevent it? I have done it before many times when the two tickets are on the same airline, but I wonder whether this is different.

Specifically, what I am trying to do: book an (international) itinerary with $ for myself on AA, then call Iberia and book the same itinerary for my daughter with avios while they correctly indicate that she is traveling with me. Rationale: miles tickets are available on IB.

Yes, I am aware of potential for complications from irops to be on separate PNRs/airlines, that is not part of the question.