This week's book giveaway is in the Agile and Other Processes forum. We're giving away four copies of Real-World Software Development: A Project-Driven Guide to Fundamentals in Java and have Dr. Raoul-Gabriel Urma & Richard Warburton on-line! See this thread for details.
Are you saying that you have two different directories that contain sources for the tests?
I will make the assumption that you either have a bunch of helper classes, or perhaps a testing framework, in one source directory and then for each of your projects you have the actual test sources for just that project, and those test sources make us of the helper or framework classes. In my opinion, the Maven way of handling this is to build the helper or framework classes in a separate project, load the JAR file into your local repository, and then reference that JAR as a testing dependency within your other projects.