One way to encourage cross-fertilization and growth of existing wikis is to provide a single login. When people request a login for one wiki, provide them with a login that works on other associated wikis, or other corporate systems such as bug-trackers, expense report submission forms, etc. Once activated, let them know that they now have access to other wikis that might interest them and encourage them to contribute there as well.
For instance, at WebWorks.com customers requesting logins for either the Customer Projects or Documentation wikis automatically get access to both. This technique has demonstrably encouraged cross-fertilization. People who have experience using one wiki are less likely to be intimidated about contributing to another, and they tend to think, “I have access, so I may as well use it and contribute.”