Ok. I will not use SHOUT int the future.

No need to synchronize yourself when you share the model (a Document
instance) between the text field and the password field:

jPasswordField = new JPasswordField();
jTextField = new JTextField();
jTextField.setDocument(jPasswordField.getDocument());

Here's a complete example:

import java.awt.BorderLayout;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JPasswordField;
import javax.swing.JTextField;
import javax.swing.JToggleButton;
import java.awt.CardLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class ShareDocument extends JFrame {
   public static void main(String[] args) {
      ShareDocument app = new ShareDocument();
      app.pack();
      app.setVisible(true);
   }

   private CardLayout cardLayout;
   private JPanel jContentPane;
   private JPanel jPanel;
   private JPasswordField jPasswordField;
   private JTextField jTextField;
   private JToggleButton jToggleButton;

   public ShareDocument() {
      super();
      initialize();
   }
   private CardLayout getCardLayout() {
      if (cardLayout == null) {
         cardLayout = new CardLayout();
      }
      return cardLayout;
   }
   private JPanel getJContentPane() {
      if (jContentPane == null) {
         jContentPane = new JPanel();
         jContentPane.setLayout(new BorderLayout());
         jContentPane.add(getJToggleButton(),
               java.awt.BorderLayout.NORTH);
         jContentPane.add(getJPanel(), java.awt.BorderLayout.SOUTH);
      }
      return jContentPane;
   }
   private JPanel getJPanel() {
      if (jPanel == null) {
         jPanel = new JPanel();
         jPanel.setLayout(getCardLayout());
         jPanel.add(getJPasswordField(), getJPasswordField()
               .getName());
         jPanel.add(getJTextField(), getJTextField().getName());
      }
      return jPanel;
   }
   private JPasswordField getJPasswordField() {
      if (jPasswordField == null) {
         jPasswordField = new JPasswordField();
         jPasswordField.setName("jPasswordField");
         jPasswordField.setColumns(20);
      }
      return jPasswordField;
   }
   private JTextField getJTextField() {
      if (jTextField == null) {
         jTextField = new JTextField();
         jTextField.setName("jTextField");
         jTextField.setColumns(20);
         jTextField.setDocument(getJPasswordField().getDocument());
      }
      return jTextField;
   }
   private JToggleButton getJToggleButton() {
      if (jToggleButton == null) {
         jToggleButton = new JToggleButton();
         jToggleButton.setText("Show Text");
         jToggleButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
               if (jToggleButton.isSelected()) {
                  getCardLayout().show(getJPanel(),
                        getJTextField().getName());
                  jToggleButton.setText("Hide Text");
               } else {
                  getCardLayout().show(getJPanel(),
                        getJPasswordField().getName());
                  jToggleButton.setText("Show Text");
               }
            }
         });
      }
      return jToggleButton;
   }
   private void initialize() {
      this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      this.setSize(300, 200);
      this.setContentPane(getJContentPane());
      this.setTitle("Share Document");
   }
}