I am having much difficulty in getting an answer that makes sense from my dental office insurance biller. I had dual insurance at the time I had services. In my experience billing medical items, we always wrote off anything that a contracted insurance provider stated we had to write off. If they had co-insurance remaining per the EOB, then and only then did we bill the secondary.
1) My dental office isn't writing off anything per the primary insurance EOB with which they are contracted. 2) The are billing different dollar amounts for the same procedures to the 2 different insurances, 3) They are trying to tell me that if the insurance didn't allow for something they can bill me more than the EOB states is my responsibility- ie. if insurance allowed for only amalgam filling vs. resin composite (I think this might be fraudulent since they are a participating provider), 4) CDA's website states only balance billing up to the lowest contracted fee, office is saying 2nd ins is controlling.

These are actually several different questions.
1. If the primary EOB states there is a write-off, the dentist is allowed to collect that write-off from the secondary. Thus, they bill the entire fee to the secondary.
2. If, after the secondary pays, there is still an amount owing, the write-off MUST be substracted first before billing the patient.
3. On amalgam vs. composite: if the insurance plan pays for a less expensive alternative (amalgam), USUALLY the dentist may charge the patient the difference between the cheap stuff and the more expensive composite material. MOST dental offices will inform the patient of this when using composite material on posterior (back) teeth. Even a PPO contract will allow the dentist to charge for non-covered procedures (such as composite posterior fillings).
4. There are literally hundreds of ways secondary insurance can coordinate benefits when the primary provicer is a contracted PPO plan. Many dentists I know of have refused to accept secondary insurance because of the confusing way insurance companies handle this. I don't blame them. In many cases, a dental office waits up to six months to get paid.

Write to me if you want more detailed information. This is sorta my area.