Originally Posted by
MSPeconomist
PNRs can never be joined, only split. It's irreversible.
Technically it can happen—they did it for me once when a phone agent issued a friend and I into two PNRs for no reason on an international reservation and we coudn't get economy comfort.
I believe they had to create a new two-passenger PNR, then exchange the old tickets for new ones in the new PNR.