There is no way to link PNRs after the fact; they can put notes in them, but they're not officially linked, and benefits of being on the same PNR (status spillover for upgrades or luggage allowance) do not accrue.
That said, I do this all the time with Master and Miss PVDProf. I buy their tickets as if they were adults. The only real issue you could run into is that some manual work on your part might be necessary to stay together in the event of irrops.