The Following Is A Bad Idea

But do you see why?

public ActionResult Update()
{
    ViewModelC c = new ViewModelC();
    if(TryUpdateModel(c, "A") && TryUpdateModel(c, "B"))
    {   
        // do some kind of update
         returnView("Result");
    }   
    // Display errors
    returnView("Add", c);
}

If you don't, comment.