title: Twoway multibinding validation
body: <p>I have this case where there is a multibinding for ex,
  textbox1 which value depend on textbox2 and textbox3 and the binding is two way,
  all textbox applied the same validation rule for ex error if minus.</p>
  
  <p>somehow if textbox1 got updated the textbox2 and textbox3 validation will fire.
  but when either textbox2 or textbox3 updated, textbox1 validation will not fire.</p>
  
  <ul>
  <li>I have tried set the ValidatesOnTargetUpdated="True" but nothing happened.</li>
  </ul>
  
  <p>Any advice?</p>