"An individual human being who wants to log in can be represented by multiple Users in Tesseral, each of which belongs to exactly one Organization."
This will be extremely confusing. You should simplify it and just keep the concept of User as we usually do. A user should have access to 1 or more organizations. That's it. You should rethink this otherwise it will be too confusing.
But isn't this kind of like saying your logins to Blizzard and Activision should actually be the same underlying user? Doesn't make sense, and becomes an authz nightmare, imo.