You are getting some terminology confused here. You can split PNRs where you have two or more passengers on the same reservation (each passenger has their own ticket even when booked on one PNR) so that their tickets are on their own reservation or grouped on separate reservations. However, you can't split a roundtrip booking into two one-way bookings (the roundtrip booking will be on one ticket even though there are outbound and return fares involved). As noted, you'll need to cancel original roundtrip ticket and then book two separate one way tickets. This is not the same thing as splitting PNRs.