The CodeIgniter framework, by default, is set up to work with a one site instance. Unfortunately, this isn't always the case and several developers have found the need to make it support more than one website based on the same codebase. Phil Sturgeon has written up this post to show you exactly how.
A few people have asked me recently about setting up CodeIgniter to run accross multiple domains based on the same codebase. This can be handy for sites than run different databases for different geographical areas, all of which need the same code but different content. To get this working I took a little code from PyroCMS and modded a previous article "How to: Support multiple production environments in CodeIgniter" and found a relatively simple solution.
You'll need to modify a bit of code to get things working: changing up the config.php to look at the hostname, making a site constant and updating the database.php config file to configure connections for each of the site constant defined.