PNRs can't be combined. They can be split but once split they can't be re-combined. As far as the agents go they are correct. Only the person who's flying can check luggage for themselves. Even if you did self service chekin they will have to see an ID before putting the tag on the luggage. Otherwise what would stop a terrorist from checking luggage under someone elses name to get it on board. You might say it would pop up when the other person checked luggage, but a lot of travelers do not check anything and would never have a clue.

flyerCO