You say you bill these separately — as you should, since they are two different carriers. Are you a third-party biller? Provider’s office staff? How did you come to be involved with this bill? If you are office staff and your billing is outsourced, why are you doing any billing?
If you are office staff, does your office request an insurance card and some form of ID from your patients? Do you verify benefits before a patient’s appointment, or at least when they show up at your desk? (If your office does not do this as a matter of cours, it needs to start doing so.) If you are office staff and your billing is outsourced, the billing agency relies on you (i.e. the provider’ office) to provide correct insurance information.
Is your third-party biller contracted to verify benefits? If so, did they verify straight Medicare, or RR Medicare benefits for this patient, or did they inform you that the patient was not covered under RR Medicare? If you work for the office of the provider that sees this patient, it is up to you to contact the patient, collect the proper insurance information, and forward that to the billing office. If you work for the biller, you need to contact the provider’s office and direct them to provide you with updated insurance information.