Confused over wp-config.php and ‘database user + database password + name’

I applied the SAME database username to both databases

Why is the Database password the same?

When I reset the password for the database username it made my main domain crash?

It sounds like you created a single MySQL user which is allowed to access both maindomain and subdomain databases. When you change the MySQL user password, you need to update the password in any wp-config.php files where that user is set.

Have I just merged two databases together? Is that even possible?

Based on your description, it sounds like you just need to update the password in the wp-config.php on your maindomain site. It is possible to make your maindomain and subdomain sites point to the same database but you would do that by setting the DB_NAME define and $table_prefix variable exactly the same.

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