Is it legal for a person to buy two health insurance?

Absolutely but expect them to have a coordination of benefits provision that requires you tell the other carrier about both plans so you don't get paid more than the expense.

Stated otherwise, the purpose of insurance is to make the insured whole, not to make a profit for either the insured or the medical provider. Therefore, policies generally contain "coordination of benefits" provisions. These can be complicated to reconcile, but in general, require the insurers to work together to pay proportional parts of the claim. The insured will not be able to recover more than the claim, and payment will be subject to the terms, conditions, exceptions and exclusions of both policies.