This is false. Each sold seat is associated with one ticket with a unique ticket number.

Each PNR can have a number of reservations for seats in it and so when the booking is ticketed a number of ticket numbers will be recorded in the PNR. However all reservations in the same PNR must be in the same booking class (fare bucket, not cabin) for each sector. So it is not possible to have two reservations for the same passenger in two separate cabins in one PNR.