title: WPF DataTemplate Trigger set a property in a different DataTemplate
body: <p>I have 2 <code>DataTemplate</code>s (A &amp; B). A contains an <code>Expander</code> and the expander's <code>HeaderTemplate</code> is pointed at another <code>DataTemplate</code> (B).</p>
  
  <p><code>DataTemplate</code> B is shown below:</p>
  
  <pre><code>    &lt;DataTemplate x:Key="ProjectExpanderHeader"&gt;
          &lt;Border CornerRadius="2,2,0,0" 
                  Background="{StaticResource ItemGradient}"   
                  HorizontalAlignment="{Binding HorizontalAlignment,
                                                RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ContentPresenter}},
                                                Mode=OneWayToSource}"&gt;
              &lt;local:ItemContentsUserControl Height="30"/&gt;
          &lt;/Border&gt;
      &lt;/DataTemplate&gt;
  </code></pre>
  
  <p>Is it possible to set the <code>CornerRadius</code> of B's <code>Border</code> when the <code>IsExpanded</code> property of A's <code>Expander</code> is set to true?</p>
  
  <p>Thanks in advance.</p>