title: unchecked wpf radio buttons
body: <p>I have two radiobuttons. </p>
  
  <p>One (<code>rb1</code>) is binded to a property of my <code>ViewModel</code>. If the property is true <code>rb1</code> is checked when the application is loaded. If the property is false <code>rb1</code> is unchecked (that's right). </p>
  
  <p>But in the last case, both radiobuttons are unchecked, and I need the second radiobutton <code>(rb2)</code> is checked when property is false. How could I do this??</p>