How do you make two 90 year old people one with dementia understand the seriousness of the corona virus that they need to stay in?

i listened to this today and thought of your question, they ask a doctor about this-so it’s a podcast specifically addressing this from medical POV : https://www.washingtonpost.com/podcasts/post-reports/feeling-lonely/
if the link gets cut it’s the Washington post April 3rd post reports- and there is a transcript you can read but it’s far better to listen and hear the emotion in these people’s voices...

As far as the elderly people are concerned for themselves alone, they may not agree that the corona virus is a calamity. They may feel a great deal more fatalistic about it than that; and if so, if your parent who does not have dementia is thinking of him/herself AND of the parent who does have dementia, it is not for you to dismiss their point of view.

There is a but. There is also the responsibility to others that they, part of society just as much as the rest of us, share.

Find a good, clear diagram that explains how social distancing works to disrupt epidemics. Show it to your rational parent. Remind him/her that this is no time to abandon a lifetime of good citizenship. Then, if they must go out, they will at least perhaps choose sensible times of day and stay away from other people.

I’ll add to my previous answer- it’s me, not him, who is struggling with accepting this may mean it’s time for him to go.

You can explain its worse than the Spanish flu. Then let them decide. They are in the 90s, they have earned that right. They have earned a say in how they lives, who they see, and how they die. For my father, who is much younger, I had a conversation about covid. At first, he honestly seemed like he was blowing it off. But after I pushed, hard-it was clear that he understood, but had made peace with his own decision. My Dad (in his 60s with Major health problems), was straightforward with me- he would die without staying social. He would spend his days largely in bed/asleep in a chair, his days would largely run one into the other, and with the reality that you can only call/interact so much over FaceTime or other virtual products, he would be left with many hours to fill- and only thoughts of loneliness and depression in supply. If left alone he would die.
Even if lockdown ended quickly, he explained, and he survived that time- the damage of being in isolation would linger physically, mentally, and emotionally; he does not believe he would recover.

Now- if it was your parents, would you forgive yourself for ignoring their wishes, for their mental anguish as they spends their last days, weeks, or months alone, for their death during that time or soon afterwards- potentially never recovering from the time you chose to isolate them?

For some, isolation is an illness far worse than covid19, and just as likely - if not more so- to result in death.

As hard as you try, you cannot reason with demented folks. If they are driving, take the keys away! If they are knocking on neighbors doors, have a talk with your neighbors about not answering the door.

If your parents have dementia, you will not be able to "reason" with them.