Here is a working solution for reusing the same JMenuBar across multiple windows: swap it upon window activation!

HotSwapJMenuBarOSX.java

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.WindowConstants;

/**
 * On OS X, with a screen menu bar, you can "hot-swap" a JMenuBar between
 * multiple JFrames when each is activated. However, there is a brief flash each
 * time the active window changes, where the menu bar disappears momentarily.
 * But it is a small price to pay to be able to reuse the same menu bar!
 */
public class HotSwapJMenuBarOSX {

	public static void main(final String[] args) {
		System.setProperty("apple.laf.useScreenMenuBar", "true");

		final JMenuBar menuBar = new JMenuBar();
		final JMenu file = new JMenu("File");
		menuBar.add(file);
		final JMenuItem fileNew = new JMenuItem("New");
		file.add(fileNew);

		final JFrame frame1 = new JFrame("First");
		frame1.getContentPane().add(new JButton("First"));
		frame1.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);

		final JFrame frame2 = new JFrame("Second");
		frame2.getContentPane().add(new JButton("Second"));
		frame2.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);

		// hot-swap the menu bar to newly activated windows
		final WindowListener listener = new WindowAdapter() {
			@Override
			public void windowActivated(WindowEvent e) {
				((JFrame) e.getWindow()).setJMenuBar(menuBar);
			}
		};
		frame1.addWindowListener(listener);
		frame2.addWindowListener(listener);

		final int offsetX = 200, offsetY = 50;
		frame1.pack();
		frame1.setLocation(offsetX, offsetY);
		frame1.setVisible(true);
		frame2.pack();
		frame2.setLocation(frame1.getWidth() + offsetX + 10, offsetY);
		frame2.setVisible(true);
	}

}