ref: http://www.contractwebdevelopment.com/drupal-multisite-configurations-an...
Issues, solutions and configurations related to multisite
I have two sites running from the same Drupal4.7 instance as explained in install.txt.
These two sites have seperate databases but share users. I did this
by editing $db_prefix in the slave site's settings.php. As explained at
http://drupal.org/node/22268.
I got errors like "Duplicate entry.." when trying to create a new user. As if the sequences table was not being shared.
A list of modules that promise different levels of User and Content Sharing across sites plus my 2cents.