Think of the different instances as similar to email domains. So you can have two totally separate email addresses, eg [email protected] and [email protected] - both have identical usernames, but are hosted from a different location.

These are two separate users, although both can email people on either domain, or indeed people from (almost) any domain.

Theoretically you could create an account on every single instance with exactly the same username.

this system is very much like email both with the names and what happens when the server goes away. backup tools are likely going to be a thing for users in the future.