CiviCRM Community Forums (archive)

Your sites can share the same database. You should use the $civicrm_setting global variable to set url related settings & also the cms reference to the users table (you should find that stuff on the wiki). You can do that conditionally in civicrm_settings.php based on the URL.

A limit of this approach is civimails as, unless you set up a domain for each site - which is the next step up in complexity - civimails that go out will have links pointing to the url the cron runs on, rather than the url they were generated from. If this is a problem for you, or if you want other configuration differentiation,  I think you need to need to consider having multiple domains (ie. entries in civicrm_domain. This is doable but there is not an automated set up process & there are some gaps in the documentation)