title: One out of two fields required in validation
body: <p>I have 2 fields:</p>
  
  <blockquote>
    <p>FirstName</p>
    
    <p>LastName</p>
  </blockquote>
  
  <p>And only one of these is required. But if both are omitted, I want both fields to be highlighted. If one of them is filled in then the model is okay and the form should submit.</p>
  
  <p>How can this be done?</p>