I tried this a couple of weeks ago with flights to Italy for my mom (dfw-fco) and me (aus-fco), the CSR said the flights have to be the same in order for our upgrades to be processed together. Granted this was for mileage + copay upgrade on an intl flight, so maybe the answer will be different in your case.

I ended up booking us both dfw-fco and will either drive up or book a separate aus-dfw RT for myself.