Setting up multiple drupals on one database sounds hard; It is not. With Drupal 6.17 you copy the default.settings.php to settings.php and start the install. If you forget to copy the file the install will remind you and you can retry or restart the install.
Once you get to the database the install asks for the database name an user account to access it. And a Database prefix name, you enter in a new name for each independent install; you can also enter the information into settings.php for the $db_url and $db_prefix examples are shown in the comments of the settings.php for the database connection.
That is it; you don’t need to make it any harder than setting up one drupal installation.
You can also install drupal to share specific database tables IE ‘users’ => ‘shared_’, you can also point multiple URLs to your drupal installation and have drupal provide a different theme for each (share code and database but do not duplicate the content) – IE share the installation but do not duplicate the content across different URLS; IE Parked sites, Subdomains pointing to your drupal install …