I´m a little bit confused and maybe from basic understanding wrong, but Is the access to different databases possible?
 
Let me explain:

My idea is to have one Odoo 10 installation with different databases.
 
In the main database I would like to build a user portal, where people can register with some personal data, which are bringing an unique key and where it ends to get proofed as a real person.

Now we  want to have additional some other databases for different projects, where we want to have the portal users there registered with Username and EMail-address AND the unique key from the main database. 

People can register in each of the project databases with different Usernames and EMail-addresses, but we know the real person behind if necessary.
 
The procedure for an new user should be the following:

1) Landing page of a project with possibility to register (or login)

2) Registering with Username and EMail-address AND all the data we need for the unique key

 --> Stored data in the project database: Username/EMail-address/unique key

 --> Stored data in the main database: unique key/Real name/DOB/etc.

Additional I would have for some projects common data on financing or logistics (same warehouses/goods/etc.)

Is that possible?