is CodeIgniter a good choice for Multi-site ?

First of all, it is very easy to make two different websites use the same database. CI has a file named application/config/database.php. You can setup multiple configurations, or two CI-installations can both refer to the same database.
Keep in mind that some hosting providers only allow access to databases from URL's on their own environment. When both your URL's are hosted by the same provider, you don't have a problem.

Next question is: can you use one CI-installation to manage two different websites? It depends. If both URL's are on the same hosting environment, they probably have their own folder inside the webspace that your provider has given you.
It is possible to put your CI-installation on the same level, rather than inside each website's folder.

Code:

toplevel folder
- codeigniter
- contentA
- contentB

In the index.php file of your contentA website, point the $system_path and $application_folder to the right position. You do the same for your contentB website. For more information: http://www.codeigniter.com/userguide3/in...index.html
To present different content to your users, depending on the url they are using, you must check the url in all your controllers.
Hope this will help you.