As there have been several questions concerning dual nationals as well as Day 2 test only I thought it might help to detail my experience this morning. I arrived from the US, my flight was early so there was no line - I think this was luck, not the norm. I used my UK passport to check in in the US, and on arrival had my covid test, completed travel forms, proof of day 2 test order and test payment in my hand. Was asked how long I'd been in the US, said I lived there and had dual nationality but was over for a few days to check up on a very elderly parent who had been isolating and living alone for over a year. When he asked for the proof of test for day 2 & 8 he noticed I had booked day 2 only. I explained that I was only staying five days - he hadn't heard that one could buy Day 2 only and I explained the rules had recently changed. I then showed him my return flight booking as proof I wasn't staying any longer than that. He then allowed me through. In spite of everything the process only took a few minutes.
Hope this reassures those who've posted their concerns about coming over as a dual national as well as those only staying a brief period of time