Why not book on the same PNR and split just before the PM upgrade window opens? That way the GM probably keeps C+/EC unless there's a late schedule that causes seats to be shuffled.

Also, AFAIK it's still an unwritten perk to be able to bring one person on a separate PNR into C+/EC with you.

You might get better advice if you tell us the route, day of week (or holiday period if applicable) and approximate time of day.