Primary vs. secondary coverage: How does it work

How to Determine Primary & Secondary Health Insurance Coverage
Primary coverage is also more likely to have generous terms and cover the full cost of the car. In contrast, secondary policies sometimes have lower limits for total coverage, lower duration of rental allowed and fewer countries in which rentals are covered.
With dual health insurance plans, one is considered “primary” while the other is deemed “secondary.” In some situations, having two health insurance plans can reduce your out-of-pocket costs. But in other cases, the added premium payment and deductible might increase your overall health expenses and cause further complications.
When you have two forms of health insurance coverage, your primary insurance pays the first portion of the claim up to your coverage limits. Your secondary insurance may pick up some or all of the remaining costs. However, you still might be responsible for some cost-sharing.