Multiple Joomla websites with a single memcached server
However, when you have more than one Joomla instance that use memcached, problems with mixed up sessions may appear.
Fortunately, there is a very simple solution - for each Joomla instance you need to set a unique secret word. You can do this by editing the configuration.php file for each Joomla installation. The exact line you need to edit is:
public $secret = 'secret_word';
Make sure you change the secret_word string to a unique value for each Joomla instance on your account. This secret word is used by the Joomla memcached controller to differentiate between the Joomla instances so that their sessions don't get mixed up.