title: Rewrite admin block that is rewriten by other extension
body: <p>I try to rewrite sales_order_view
  Here is what I have in config.xml</p>
  
  <pre><code>        &lt;blocks&gt;
          &lt;adminhtml&gt;
              &lt;rewrite&gt;
                  &lt;sales_order_view&gt;Bitstream_FancourierSelfawb_Block_View&lt;/sales_order_view&gt;
              &lt;/rewrite&gt;
          &lt;/adminhtml&gt;
          &lt;bitstream_selfawb&gt;
              &lt;class&gt;Bitstream_FancourierSelfawb_Block&lt;/class&gt;
          &lt;/bitstream_selfawb&gt;
      &lt;/blocks&gt;
  </code></pre>
  
  <p>Ofcorse, I have the proper file in Bitstream/FancourierSelfawb/Block</p>
  
  <p>All I need is to add a button but looking over other modules I see that block is already overwritten.</p>
  
  <pre><code>        &lt;blocks&gt;
          &lt;adminhtml&gt;
              &lt;rewrite&gt;
                  &lt;sales_order_view&gt;Fooman_EmailAttachments_Block_View&lt;/sales_order_view&gt;
              &lt;/rewrite&gt;
          &lt;/adminhtml&gt;
      &lt;/blocks&gt;
  </code></pre>
  
  <p>If I comment this in config.xml from Fooman module I can see my button. Otherwise, no luck.
  Is there a way to overwrite same block twice in different module?</p>