"My husband is just afraid that she will throw away the card and not use it."
So what if she does? How would this have any impact on dad if she chose not to use his insurance? Presumably, if mom had medical procedures done that resulted in unreimbursed expenses that dad's insurance would cover, he could file a claim to get those expenses taken care of. Dad's getting a credit for having insurance, so why does he care if the child is carried on stepdad's insurance?
As for who's primary, and who's secondary, that is likely up to the insurance company, not mom. My insurance company assigns primary/secondary coverage according to the birthdates of the insured parties. My ex's birthday is before mine, therefore he's primary (even though his insurance is in a state that our children do not live in and therefore is of little use to them as all providers are out of network, therefore mine always covers it).
Also, it may not cost the stepdad anything extra to have the child on the policy. Mine is a certain premium cost for an individual, then another premium cost for a family, it makes no difference whether there are 2 people or 10 on the family policy.
"Let me ask you this, if the daughter gets hurt at our house, or her moms, do they still split the bill 50/50? The only thing the order says is that they both pay half."
Irrespective of where a child gets injured, both parents are responsible for paying the unreimbursed portion of the med/dental according to the decree.