Is it a good practice to have 2 SQL Severs for a SharePoint site? If yes, can you recommend me link(s) for how to add it?
If you want to split your site collection into multiple Content DB and then spread those DB across both SQL server. then it is Big NO
You can not have a site collection in to Multiple database, it is not supported.
If you want multiple SQL server attached to a farm, then you can do it. It is supported. we have 4 SQL server attach to one farm due to content. Usually customer split between content and services & config dbs. If you have high traffic sites on single sql server which will cause you performance issue, then adding another SQL server is best option.
To configure the 2nd Server for the content, you dont need to perform anything special just main requirement
- set the permission for Farm Admin Account
- Create an Alias for the new Server
- Create new DB on this server
- follow the instruction here : Best practices for SQL Server in a SharePoint Server farm