It seems that in the past on CO you always wanted to be on a split reservation to get upgrades, while on United you always wanted to be on the same reservation.

I was looking at the new buy-up to first today (while reprinting a boarding pass), and noticed that it states that "All passengers checking in will receive prices based on the highest Elite status in the reservation"

If you were traveling with a SO on split reservations, and saw that your odds didn't look good (after checking in) and decided to pay the "Tens of Dollars" the fact that your SO was on a different reservation would make that upgrade more expensive.

I know that this wont happen often, but it certainly looks like CO is (slowly) moving in the United direction of flying on the same reservation (which actually makes more sense).