That is my understanding also. You can have more than one person in a PNR if they are travelling on exactly the same itinerary, you can't if they are on different itineraries. As you said you can divide a PNR if someone needs to change their itinerary, but you can't combine two PNRs with different itineraries. You can LINK them - but that's simply a case of putting a note in each PNR to indicate that the passengers are travelling together.

(All the above is based on what I learnt when I was working for an airline in the late 1990s, as far as I know things haven't changed but obviously I'm willing to stand corrected!)