I've been looking into this issue as well, and attempting to come up with the best solution for a client request.
There are a couple of ways to achieve this, such as the dummy/fake emails, and create a secondary email field. Which will not trigger WP's unique email requirement. The new field can simply be labeled 'email2'.
'email2' can contain any information you want [limited by MySQL field definition]. Once it's created, add the same email across both accounts (or many multiple in my case).
Then in your code, make sure both accounts are mirrored for actions/triggers/updates etc...
For example, if account 1 has an automated message, and/or an account update etc... Your code could reference 'update all accounts where email2 = email2 of the current logged in WP_user'.
It's a work around, but should satisfy most client requests. They won't see what's going on in the backend. But when they login, they should see a 'combined' or 'mirrored' view of both accounts. Because, in essence, they are the same other than WPs unique email, and username requirements.
You can even do this with passwords. You cannot 'mirror' the passwords. However, you can reference and update/set the password, of all accounts where email2 = email2 of current logged in user [wp_set_password].
Also, there is this WP plugin [not my plugin FYI]:
If anyone else has any other solutions, let's brainstorm and figure out the best solutions. Are there any better solutions? #StackExchangeIsCool