You cannot combine two PNRs but a remark can be added in each booking to cross refer them, no need to cancel and rebook, that sounds very risky. There is nothing more confusing than cancelling and rebooking exact same thing. This cross referring is called a To Complete Party TCP. advice, its more than just a remark, its an OSI. This can be added to two or more bookings made at different times for the same flights for the same party or bookings that have been split out, maybe one needs to return a different date for example but still need to cross refer the outward for seat allocation purposes. The res agent probably cross referred the bookings without you knowing, as that is what a good res agent would do.