Does natural parents insurance trump a stepparent no matter what?

I am divorced and single, my ex is remarried. This question has to do with medical coverage for our 2 children. I carry medical insurance for my kids, as does his wife. Our divorce decree states that I was to continue coverage for our kids on my insurance while I remained employed with the company that I was with at the time of the divorce. It also states that if my employment changed, then we will share in the expense of medical insurance. I changed employment and have coverage with my new employer. I have been asking him to pay his half since the decree states that we will share in the expense. He is refusing, stating that our kids are covered under his wife's plan, although it is my insurance that he uses when he takes our kids to the doctor. I have a court date to argue whose insurance should be deemed the primary insurance for our kids, mine or his wife's. My birthday falls first in the calendar year to both of their birthdays. Could a judge order, even though I win by the birthday rule, me to pay half of her coverage simply because my insurance is more expensive than hers?