title: MVC 3 field mandatory if another field is filled
body: <p>I have a simple question.</p>
  
  <p>I have for example two fileds mapped on model ex: textbox_1 and textbox_2.</p>
  
  <p>I whant to ask if exist a way (ex mandatory decorator) that imposes textbox_2 mandatory ONLY if I fill textbox_1. IF I do not fill textbox_1 textbox 2 is optional.</p>
  
  <p>Is there an elegant way to do this ?</p>