I agree though that the amount the NCP pays should be figures soley on what is the patient responsiblity after both insurances have paid as listed on the Explanation of Benefits statements.
You could try to look up your state law on coordination of benefits (there is such a thing calles COB savings that could come into play) and get copies of the contract language in both contracts. Go into the providers office if need be to explain it and negotiate with them to stop requiring the payments up front.