Can two people be signed in under one account?

Mac yes. iPad and iPhone are meant to be one user devices and not designed for switching accounts on a regular basis. On Macs you can have two separate user accounts. On iPad/iPhone you would have to sign out the other user and remove all the data which is not practical

On a Mac, yes. On an iPhone or iPad, no.

Yes but you are asking for trouble.

You mean if the same account can be signed in on two separate devices used by different people? Yes, though it's not recommended practice.

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