Reservations can't be linked, but the RA can add a notation to each that there's another person(s) travelling w/ you.

If one of you has 50K status (gold may simply mean Star Alliance gold) w/ UA then one person + companion can be in E+. Gold status on another carrier gets more baggage and on the plane a little sooner, but no E+.

Suggest going early to airport - ask at check in, ask at the gate, ask the FA - etc. If one of you has an aisle or window, it's likely one passenger will switch (mention the projectile vomiting that occurs when the SO isn't there to calm the child, or whatever). Might help to call again, ask for a supervisor - but be cautious & cordial, you don't want to get "flagged" as a difficult customer.

Suggested approach is NOT- UA have screwed up again, but rather, we're travelling together and altho we're on different pnr's is it possible to sit together?

Hope something works.

(Have had many lost seat assignments on UA - always when there's been an equip. change, many times when there's been a mild change in schedule or the stars didn't line up properly. As flight time approaches, check daily. )