When I read the topic title, I thought it might have to do with the following example, which I have considered, but have never actually done:
(1) Buy a RT ticket LAX-LHR.
(2) Buy a RT ticket LHR-BCN with same departure and return dates as flight above.
(3) Remove staples from all tickets and arrange the four coupons and staple back together to give the impression that the two tickets were purchased as one itinerary.