Best approach for MySQL user tables, and SQL Server data tables

Right now I’m trying to understand the best approach for doing what I want to do. I have not tried anything yet but hoping to get some direction.

I will be using Joomla and MySQL on my server for user creation, maintenance, etc.
There will be a button on Joomla site that will run my application via url like app.mysite.com

I can take care of passing an encrypted or special parameter that would allow me to know in my SC application which user is running my app. Don’t worry about that part.

My question is my app will have to select sum data from the joomla MySQL tables, but my application will mostly use SQL Server tables for all the other data.

I haven’t used SC in quite a while so may not be up to speed on newest versions, but what is the best way to connect to multiple database types in this scenario?