Locum Tenens does NOT apply to NP/PAs. You bill for their services exactly as you would any other NP/PA.
So - IF it meets incident-to (established pt, established problem, established plan of care, physician onsite), AND the carrier follows incident-to, you may bill under the onsite physician. If it does not meet incident to or the carrier does not follow incident-to, then it is billed under the NP/PA who actually provided the service, unless the carrier has a specific rule stating otherwise, and then bill according to carrier's written policy.
You may not bill your temporary NP/PA under the physician who is out. Probably not the answer you wanted to hear, but you will need to credential with insurance for these NP/PAs.
PS - CMS officially now calls it "Fee-for-Time", but it means the same as LT.