My husband currently has 3 medical insurance plans. Medicare is primary, Medicare supplement is secondary, and then he is part of a special insurance plan that will cover everything the others don’t for specific conditions only.
Providers never get it right and when they don’t submit in the right order with the right codes things don’t get paid.
From my personal experience, having multiple insurances (primary and secondary) can mean having to spend time getting the billing done correctly by your health care providers. YMMV