I was on two PNRs with my family in 2006 or 2007. My wife and kids flew together but I had a shorter vacation. We returned HKG-SFO together. The check-in agent was prepared to op-up me, but when she saw I was actually part of a group of four, she called inside and offered me an op-up for two (although both I was alone on my itinerary).

I was on vacation in HKT two years ago. We were traveling together on one PNR, but I was offered an op-up and asked if I would take it.

I was seated in the J cabin on a Y ticket to TPE about two years ago. I had a travel companion on a different PNR. When I asked to re-seat myself in the Y cabin, they seated my travel companion in the J cabin instead.

So combined or separate PNRs don't matter much.