Thank you all so much for your time and help. My situation is a

1. Both dental plan's coverage are $2000 regardless it's primary or secondary. I discussed this primary/secondary question with their customer service department.
2. I did let dentist know we have two dental plans. Otherwise he can not claim.
3. The dentist did return $800 to my primary insurance company. I could not understand either.
4. The treatment had been completed almost 2 years. We have given him enough time to return the deposit.
5. We did not have any problem with him before we requested the deposit.

Anyway, just let it go. Thank you very much again.