You would need to have opened ports at your origin (3306 if default) and allow IP address for each user (or more of them) from each “wordpress install domains” to connect to other one, also knowing the “master” or at least “the one” where the database is hosted, so using the same username and password for MySQL you can connect.
DB_HOST with the value of
localhost you would write either your
hostname or use direct an
IP address to connect to your “the one” (where the database is) from others.
That way, separate WordPress installs would use the database on “the one” using correct credentials.
The thing can be more complex if using secure mysql, so therefore you would need to setup an SSL for your MySQL and use it thatway.
Later on, you can setup WordPress multisite and for easier manage of all of them either like that or using ManageWP.com.
This topic on WordPress support can help you:
Furthermore, hope you do not have a cluster, where you run “one master” MySQL and two or more “child” MySQL instances, each of them on a separate server and different IP address. That would be a really complex system at a first sight to manage at least.