NPs and/or PAs can only bill incident to as it relates to physicians. Even if your other NP co-signs the notes, it is not permissible to bill under another NPs credentials. The NP you want to bill under would have had to actually see and examine the patient.
Different practices have different ways of dealing with the situation you describe. Options include:
1) Your new NP basically shadows your existing NP, but does not see patients independently
2) Your new NP sees patients independently, but only those that can be billed incident to the physician (established pts with plan of care in place, etc.)
3) Your new NP sees patients independently and you bill accordingly under the new NP, with your office aware that claims may be denied by insurance.
4) Your new NP sees patients independently and you do not bill for the services.
5) Your new NP only sees patients with insurances for which he/she has already been credentialed.
Or any combination of the above depending on your practice's needs.
How you decide to do this is basically a business decision.
And now as I re-read your question, when you state "credentialed" do you mean they are not licensed yet?? If so, I don't believe they may independently see and treat patients. Different states may have different requirements about that. If that is the case, I would only be comfortable advising option 1 above. I would also imagine even if they may legally see & treat patients under the supervision of another provider, it cannot be billed to insurance, just as you cannot bill for a resident or fellow's work without the teaching physician also personally examining the patient.
When I use the word "credentialing", I mean enrolled as participating in a particular insurance.