credentialing

1. How many specialties can a DR can be credentialed for.
2. If they can and they have both certification in IM and Neprhology for instance, can two docs in the same office (IM) see the same pt same day in the hospital and be paid if one is Im and one is Nephrology...
3. How can you dicern when you bill that dula DR as Im and Nephrology?

Thanks for feedback to help with a debate...