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To add to Amma's great response, I think you should try to engage with both, but focus more on the person who is not taking the notes, as that is the person who you will likely be able to hold more eye contact with. Also, at least in my experience, it tends to be a partner/counsel leading the interview with questions and an associate taking the notes. Since the impression you make on the partner will likely influence your progression chances more, you want to concentrate on creating the best rapport with them.
