Right. Lets see what happens when i try to check in online with the unitd app. First hurdle is that UA has not seen the visa yet, but on other flyertalks threads it seems as if you can actually successfully add visas by scanning through the mobile app. But will it take the second (US) passport as acceptable option for the exit tracking of the DE passport.
And if i may digress: USCIS made me collect 5 years of international trips data (50..110kM/year) for naturalization and the interview officer actually checked all the stamps on my DE passport but was happy to stop seeing more since i became GE three years ago (no stamps). Aka: I would be scared (bureaucracy incompetence) if USCIS could not access CBP entry/exit data, but i am a lot more annoyed assuming that they can access the data but still had me run through hoops writing down this data myself (sure i did plan for this and had "heap of boarding passes" ;-). As in: we just wanted to give you one more option for failing naturalization by letting you write down details no human should need to remember. Pure harrassment.
Can you sue a bureaucracy for harrassment ? Wait: wrong question. Can you succeed sueing a bureaucracy for harrassment ?. No chance in DE, but maybe US is better ? ;-) (yes, ok. i will wake up now and be docile again).
But its kinda on subject: If CBP exit data on DE passport comes from check-in data and is passed to CBP, how actually does the trusted traveller number (in my UA profile) influences what info is needed during check-in ? UA must be able to get feedback from CBP "valid trusted traveler, print TSApre on BP". Would UA also get a bit back from CBP - "US citizen", DE passport sufficient for check-in ?
So i definitely want to go to CBP/GE and have them add my US passport to my profile before trying the UA online app to check-in online. Lets see.
This is what i meant with "what happens under the cover".