If divorced parents both carry insurance and father's is primary can the mother file hers as primary?

Just file and when the question asks about other coverage - answer it honestly. I see so many questions on this site about primary - secondary and don't understand all the confusion. Just fill out the forms and wait for payment.

I've search all over the Insurance Code http://www.leginfo.ca.gov/cgi-bin/calawquery?codesection=ins&codebody=&hits=20

and Westlaw and haven't found the rules on which company is primary, other than Medicare in groups of over 20. I've been in the business 20+ years and haven't found it to be a real issue.

The birthday rule applies here. If the fathers month and day of birth falls before the mothers than he will pay as primary. If the mother submits a claim and the insurance company pays as primary and it turns out that they were not, the will and can ask for that money back, up to and including the involvement of a collection agency.

Most insurance companies have an automatic system that automatically flags a dependant that may have the possibility of other coverage.