Are these when you are on the same PNR? Some have speculated that it's based on alphabetical order in these cases. What's the first letter of your first names? Otherwise, additional tie-breakers (aside from ones you mention) in order are Fare Class (which comes before Amex Reserve cardholders), Delta Corporate Traveler, Amex MQD Waiver qualification, and date/time of upgrade request (usually time of purchase).

https://www.delta.com/us/en/skymiles...llion-upgrades