Keeping in mind the difference between PNRs (reservations) and tickets, in the old manual days, it was possible to have one ticket that had coupons for flights in different PNRs. An agent would create different PNRs, issue a manual ticket, and update the different PNRs to contain the manual ticket number. I'm not sure how it would be done in today's automated world. Ignoring limited-changes PNRs that are automatically issued due to flights on multiple airlines, code-shares, etc., since those are not germane. It might be possible using low-level SABRE or Amadeus. But it's a theoretical question with no practical application.