Is there a way to share the same *.java files between Netbeans and Eclipse?
Sharing the Java source files is not the problem - just put them into some source control system, and then import them in both IDEs. Usually the configuration, especially the build process, is the main problem.
In practice, the solution is likely that you maintain separate project configuration files for both IDEs, but use a common build script (e.g. ant) that is supported by both. The project configuration allows you to use IDE-specific features, while the build script makes sure that both environments produce the same output.