Is there a tool to connect the databases of 2 sites sitting on separate networks with one login allowing them to share a common user base?
I have searched around, but I unable to find a clear answer or a way to be done.
In case if you are looking for a single sign-on (SSO) solution, take a look at onelogin (paid of course).
A workaround would be to use oauth based sign-up allowing logins which are already centralized (a bit similar to how stackexchange sites work).
either, there is one solution which I have not tried yet as I also want to achieve the same. What you can do is you can implement any plugin for user registration, but then you have to mantain the wp-users table in database yourself.
For ex. If Site A and Site B are there and user registers on Site A then you have to manually take entries from Site A's wp-user table and put it into Site B's wp-users table.
Someone told me that there is some - Crone Job Script that can do this for you after a specific regular interval of time.
PS. I asked the same on Quora a while ago, where someone answere this for me.