Originally Posted by
SEA1K4EVR
To clarify for the OP... at the airport on the day of the flight they can clear separately. It's just in advance that they must clear together.
This is interesting. If a SWU is used on a flight that is typically a battlefield upgrade, say HKG-SFO, connecting from SGN-HKG, this might not be as big a problem.
I do agree that booking flights as single entities is always the better move if you're desiring to upgrade.