For "incident-to" billing of a service :

There must have been a direct, personal, professional service furnished by the physician to initiate the course of treatment of which the service being performed by the non-physician practitioner is an incidental part, and there must be subsequent services by the physician of a frequency that reflects his or her continuing active participation in and management of the course of treatment.  In addition, the physician must be physically present in the same office suite and be immediately available to render assistance if that becomes necessary.

In your question, the NP did the initial service so the subsequent service would not be billed as incident-to.  It would need to be billed under the NP's provider number as far as Medicare is concerned.