Another thing to think about, and I'm not sure if this was the original intent of the question, but I can read into it if I try hard enough...
If this is an established patient, and the NP is seeing the patient for an established problem and an established plan of care, and "incident to" is being followed, you bill under the supervising provide that day. It doesn't have to be billed under the original provider that first saw the patient and established the plan of care. So in essence, a different provider could be supervising today and has never seen the patient.
For this type of group practice, the provider that is supervising is the one that is billed on the claim, regardless of whomever saw the patient originally, and as long as "incident to" is being followed.