As you said, just get separate one ways is one direction and on the return have you pay for it on 1 purchase and have him reimburse (this way all upgrades are together and on one PNR). Anything else won't be quiet as good. You won't get a merged PNR. Using either certificates or points won't guarantee any upgrade. If you have the status and/or card, you can get the person in the lounge but that's it.