Perhaps the greatest strength of the World Wide Web is the ability to easily embed links from one piece of information to another. In fact, it’s this very behavior and the resultant cross-linking that gives the web its name. This same philosophy also applies to wikis, after all a wiki is an editable web page and therefore supports the same sort of hyperlink technology that makes cross-linking possible. In addition to using hyperlinks in your wiki to link to additional content and information on traditional websites, also use them to link to other wikis.
Including links from one wiki to another will help make the community members more aware of the other wiki (or wikis) that you wish to promote. With increased familiarity will come an increased awareness of the value they can add, and a desire and willingness to contribute to multiple wikis.