Do you mean on the LHR-JFK leg one person will have two bookings - an award in F and a revenue ticket? If so, might that cause trouble if the passport number is entered on both bookings? And presumably it will already be on the round trip, as they'll have used the outbound and filled out the API.
That's the case. I'm trying going on a whim and assume everything will be ok if we tell the airline that they won't be using their Y ticket (it's a kid anyway). Just wanted to see if anyone had any experience with this.