Accessing UserControl's DataContext from another UserControl

Given these classes:

public partial class MainPage : UserControl
{
    public MainPage()
    {
       InitializeComponent();
       //...

    }
}
public partial class LoginForm : StackPanel
{
   private LoginRegistrationWindow parentWindow;
   private LoginInfo loginInfo = new LoginInfo();
   public LoginForm()
   {
       InitializeComponent();
       //...
   }

   private void LoginOperation_Completed(LoginOperation loginOperation)
   {

    if (loginOperation.LoginSuccess)
    {
            // Here I need to access MainPages's DataContext property and set it with my ViewModel
    }
   }
}

I want to set MainPage's DataContext property inside LoginFrom. So I created the static instance of the MainPage class in the class itself:

public partial class MainPage : UserControl
{
    public static MainPage Instance { get; private set; }
    public MainPage()
    {
        InitializeComponent();
        Instance = this;
    }
}

Now I can access to the MainPage's DataContext this way:

MainPage.Instance.DataContext = new NotificationItemViewModel();

Now I'd like to hear your ideas about this kind of passing data between classes in a Silverlight application. Do you have any idea?

PS: actually I answered my original question here on Stackoverflow. But I'd like to hear your ideas in terms of code review.