Use Same User Data on Multiple installs of WordPress

First off let me say this isn’t a WordPress multisite question, I do not wish to use WordPress Multisite in this scenario.

What is needed the ability to use the same User data (profiles, login data etc) on 2 or more single in stalls of WordPress.

I also want to clarify that the databases will be on the same server. I would like to keep databases separate for the installs but using same database isn’t out of the question.

Now with those rules laid down lets say I am using the domain name abcdefg.org
The server directory will see two installs of WordPress one in the web root directory
/public_html
and another in a sub-directory
/public_html/subdirectory

So that is two installs of WordPress, in this case, while it really doesnt matter about the themes, the theme for the web site visitor will look the same, as if they are on the same site. If they need to login to the site they will be logged into BOTH installs of WordPress so they can travel seamlessly from one install to the next, as to them this is one site that looks the same.

Other considerations, once a new user registers for the site, they will get only one email (or the admin will only get one email) and depending on which WordPress install they registered on their registration will sync with the other install of WordPress.

Keep in mind that this could include multiple installs of WordPress there could be 3, 4 or more if needed for a “Super Site”.

Now my question is, does anyone know if a setup like this has been achieved and if so where could one go find documentation on a install like this.

Thanks for your help.

Roy