When someone is covered by two health insurance policies, one is designated as the primary health insurance plan, and the other is designated as secondary insurance. Health claims are submitted first under primary insurance.
The secondary health cover insurance is for the better facilitation of the individual. What is covered by the primary or the first health cover is paid by the insurance providers, and the rest by the secondary insurance provider what was not covered by the first/primary health cover provider. Employee’s plan provided by the employer is considered as the primary one even if the second one is better, but claims are to be lodged with the first.
Not as a law, but by insurance practitioners practice so, and that is the birthday rule. The children under age are covered by parents’ health covers but the primary source, in that case, would be the parents who are elder – having birthdate older or first; and the secondary plan would be treated of the other parent who is younger in age, or the birth date comes after.
By the law of the ACA - Affordable Care Act, married and unmarried children up to the age of 26 can be covered in health insurance by the parents. In that case, the university, school, etc., would be treated as a primary plan and the parents’ secondary health cover insurance.
Spouses, either married to or in a domestic partnership agreement, can be covered by health provision, but their plan would be treated as primary, and one of the partners as a secondary plan.