Joint health insurance plans when you and your spouse retire

Hi, I’m not close to retiring in my life yet, but my parents are this year. They both will be able to receive Medicare for health insurance when they retire, but they are also looking into getting a secondary health insurance for things that Medicare wont cover. They went to an agent recently who gives advice on what to expect, how other health insurances work when you retire, etc, and they came back and told me about it. One thing that I thought was weird is this:

When you are both are married, and both retire, you cannot have the secondary health insurance cover both people; no joint accounts. Both married individuals must have their own separate health insurance, plans, deductibles, etc. I’ve never heard of this before since I’ve never read up on retirement health insurance in my life, but is this actually true? I just wanted to ask since it means you are paying double for two health insurance plans with you and your spouse, besides both having Medicare.

Thank you for any help! Also sorry if it’s a silly question, maybe this is common knowledge, but I just never heard of this and didn’t know if this is a state by state thing, or the whole USA when it comes to this.