I have two dental insurance plans. Why do I still have to pay a lot for the procedures that I did?

I have dual dental insurance. One is under myself and the other one is through my spouse. I just did some fillings and deep cleaning with my dentist. Both insurance companies covers 80% for those procedures. The dentist is in the network of my primary insurance which is under myself, but not in the network of my secondary insurance. For example, my dentist charges me 200 for a filling. Since it is in the network of my primary insurance, they only allow him to charge 100 for the filling, and the insurance company paid 80 for it, leaving 20 to my responsibility. Then the dentist charge the secondary insurance 200 again. The secondary insurance covers 80%, with the coordination from the primary insurance, they paid (200 * 0.8 - 80) = 80. Now my dentist says that I still own them (200 - 80 - 80) = 40. Is this right? It sounds like if I end up paying more with dual insurance than with single insurance.
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