You can only get on the "airport upgrade list" when you are at the airport where you want the upgrade [Someone - please tell me that I am wrong - I would love to know for future reference]

For future reference, you would have had a better chance if you had both segments on one ticket. The "time" for upgrade (72 hours for platinum) is as of your first segment, but you can get upgrade on your second segment. So, you would have had a 2 hour advantage over people who are originating in DFW.