If you are par with the insurance. Then adjustment would be taken on the basis of which inusrnace has hight allowede amount.
Eg:- Prim Insurance Aetna : $100 bill : insurance allowed $75,
Sec Insurance Cigna : $100 bill, insurance allowed $80,
In this case , will post $ 20 as w/o, Cigna will pay $5 addition to Aetna's payment.
Thanks,
Amit