Single or Multiple Databases
For any database driven website, the database is a backbone that must remain secure. That now leaves the question whether or not to use one database or multiple databases? The school that invented the first Apache server, first website and is still amongst the top computer programming schools in the United States had me convinced that more databases is a better option since it made for better overall security of data; especially if backed up elsewhere. Personally, I like one website to have at least one database. One is often more than enough. But, if you decided to use 2 frameworks for 1 website, 2 could be the better option. Multiple databases is one area where Drupal excels. Some people like using 1 database for their various installations. One downside to this could be that someone with access to the database could alter it in such a way that it ruins things for everyone. Imagine every client not being able to login. If a clever hacker had you make his website with Drupal and he requested a backup upon payment, you might want to think about him having his own database since the connection username, password and database name may be the same, especially with shared hosting.
Anyways, like a good boy, I believed that all files, 1 database with specific passwords per site and backed up sql files and a good log file is quite good to ensure the sites can stay up and running. Keeping all the files in the public_html folder for each website with the databases can allow for quick recovery. And, always expect the worst so that you can fix the mess if you must.