Is there a way to share the same *.java files between Netbeans and Eclipse?
Of course. eclipse supports linked source folders. The sources files don't have to be located in an eclipse project folder.
Inside an eclipse java project, select New -> Folder, then select "Advanced" and check "Link to alternate location" (eclipse 3.6, may be slightly different on other eclipse versions). "Browse" to choose the source folder from the netbeans project, press Finish.
Now you have linked (standard) folder to the Java files inside the netbeans project and modifying those files will modify them at the remote location.
Finally - right-click on this linked folder and choose Build Path -> Use as source folder.
(I leave the other way round to the netbeans experts)