As admin you can define sites that can be used as external Wikis. From the admin menu click on the External Wikis link to get to this screen:
In this screen you can add any number of external Wiki sites. You have to enter a name (no whitespaces, please) and the URL in the URL field. The special $page syntax must be used to place the name of the page in the URL.
Once the external Wikis are set up you can use this syntax to link to an external Wiki:
For example WikiName:PageName would be replaced by a link to the URL for WikiName and the specific PageName Wiki page. Something like: http://tikiwiki.org/tiki-index_raw.php?page=PageName
For more information about a common map, see InterWiki on Meatball Wiki.