Billing entities can hire any number of employees to carry out the work. They can then bill as though they did the work themselves. So - Billing entity's Type 2 NPI Number for who gets paid. Type 1 NPI Number for who did the work. In this instance, it would be the Type 1 NPI Number for the supervising physician.
If the supervising physician is the one who is getting paid, then both NPI Numbers belong to the supervising physician. If both the supervising physician and the RN are employees of a business entity, then the Type 2 NPI Number (who gets paid) would be that of the business entity. The Type 1 NPI Number would be that of the supervising physician.
There is a more thorough discussion of this issue somewhere in these threads - but I can't locate it at the moment and don't have time to search more thoroughly.