Database Dilemma – Two usernames, one website

I have created a custom template containing a PHP file which connects to a database other than my normal wp database. I know my coding is right because this file works outside of wordpress. The trouble (I believe) is that when opened in wordpress my wp database uses one username and my other database with my host uses another. Even though my php/sql file calls for the correct username for the host-database, I can’t log on because my wp-config file overrides and uses its default username. The trouble of course is that when I change the wp-config file, nothing on my site works at all. How do I correct this?