If you and your partner are covered on each other's health insurance, you don't get to double-dip on benefits. You can't file the same bill with both insurers and get paid twice, or get twice as many annual checkups. Whenever you, your partner or your children go to the doctor, you have to determine which insurer is primary. Secondary insurance will kick in as appropriate to help handle the uncovered portion of the bills.
Primary Health Insurance
When you need a doctor, you can't pick between the two insurers based on which offers better coverage. If it's your broken finger, say, that you want treated, your insurer is always primary. Likewise, if it's your partner who needs an antibiotic prescription, her insurance pays primary coverage.
Depending on the scope of the coverage, the secondary insurance may not kick in at all. It won't play a role until the primary insurer has paid as much of the bill as it can.
The Birthday Rule for Children
If your children are covered under both your plans, what insurers call the birthday rule kicks in. Your insurance is primary for dependents if you're born earlier in the year; if it's your partner, her insurance is primary. It doesn't matter how much older one of you is than the other – only whose birthday date is first on the calendar. If the two of you have the same birthday, whichever one has been covered longer gets to be primary.
Exceptions to the Birthday Rule
The birthday rule doesn't apply to every family. If your partner has a group plan and you have an individual policy, your spouse's plan is primary for the children. If you have regular insurance and your partner is getting by on COBRA, it's your plan out in front.
In a divorce, whichever of you has custody is primary. If the custodial parent remarries, his partner's insurance is secondary, and the noncustodial parent's comes in third place.
Secondary Health Benefits
If the primary insurance covers a bill completely, the secondary policy is irrelevant. It's when the primary insurance leaves some of the medical bills unpaid that having two policies pays off.
Suppose your child goes in for $3,000 worth of dental work and the primary policy has a 50 percent copay. That leaves you with $1,500, but you can look to the secondary policy to help with that. Secondary health insurance will pay up to its own policy limit, but you can never recoup more than the actual medical bill.