Pseudo "subdirectory subsite" While a real subdirectory subsite has a URL of example.com/site1, and shares codebase with example.com, I was wondering if it's possible to create a separate Drupal installation (in a separate docroot) for site1 and still use the URL example.com/site1. In my case, they are two completely different Drupal installations with nothing to share, well, except the somehow 'related' URL part. 

I know I can just use a sub domain and make the whole thing a lot easier but am wondering if the pseudo 'subdirectory subsite' is an option.
I think you could use an alias in your apache virtual host, something like this

Alias /site1 /path/to/your/site1
&lt;Directory /path/to/your/site1&gt;
  Options +Indexes 
  AllowOverride All 
&lt;/Directory&gt;


Like it is suggested here https://stackoverflow.com/questions/19055372/redirect-to-a-subfolder-in-apache-virtual-host-file#19055573