title: MVC 2: Cross field custom validation
body: <p>I need to do some validation where 1 of 2 fields must be entered.
  What is the best was to do this in MVC 2?</p>
  
  <p>The fields are;</p>
  
  <pre><code>&lt;%: Html.EditorFor(model =&gt; model.contract.ClientOrderNumber)%&gt;
  </code></pre>
  
  <p>and</p>
  
  <pre><code>&lt;%: Html.TextAreaFor(model =&gt; model.contract.InstructionWithoutClientOrder, 
                          new { maxlength = "255", style = "width:200px;height:100px;"})%&gt;
  </code></pre>