On the Millwood Online blog there's a quick step-by-step about getting multiple Drupal sites up and running on one code installation.

One of the great things about Drupal is that you can run more that one site off one installation. This saves disk space and load on your server, it also saves maintenance time. With only one installation to manage and update the time spent administering your site drops greatly.

The process is pretty simple - install Drupal normally and set it up for the main domain, then change the directory name for the settings and add another for the new domain name. Go update your Apache virtual host configuration file to point to the new location and restart the web server. Both of your domains should be set and ready.