If you use Application Designer in design time mode ( i.e. using the Application Designer class loader), then place the classes for each project inside the project's /appclasses/classes or /appclasses/lib directory.

Make sure that the same class does not occur in different projects. The Application Designer class loader embeds all classes of all projects in one view and loads the first class it finds. Keep in mind: if turning to runtime mode (i.e. class loading of web application class loader), then all classes have to be transferred to /WEB-INF/appclasses/classes and /WEB-INF/appclasses/lib anyhow, and having two class implementations holding the same names also will cause conflicts at this point of time.

For detailed information, see Appendix D - Class Loader Concepts.