Xaml Binding outside itemtemplate - silverlight

Question

I would like to bind data of my listbox. Imagine I have something like :
<ListBox ItemsSource="{Binding MyList}">
<ListBox.ItemTemplate>
<DataTemplate>
<TextBlock Text={Binding Value} />
<TextBlock Text={Binding AbsoluteValue} />
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
MyList contains an observable collection of an object that has a property named "Value"
AbsoluteValue is a property of the view model, as the MyList property.
Of course, the seconde textbox will have always the same value, but it is what I want :)
How can I tell the binding that the datacontext is not the same for the second textbox ?
Thanks in advance for any help
EDIT : my real sample is a StackPanel.
I've tryed
private void StackPanel_Loaded(object sender, RoutedEventArgs e)
{
StackPanel stackPanel = sender as StackPanel;
stackPanel.SetBinding(StackPanel.VisibilityProperty, new Binding("Loaded") { Source = DataContext, Mode = BindingMode.TwoWay });
}
but it's not working

Answer 1

XAML:
<TextBlock x:Name="tbAbsoluteValue" Loaded="AbsoluteValue_Loaded" />
Codebehind:
void AbsoluteValue_Loaded(object sender, RoutedEventArgs e)
{
TextBlock absoluteValue = sender as TextBlock;
absoluteValue.SetBinding(TextBlock.TextProperty, new Binding("AbsoluteValue") { Source = VIEW_MODEL_OBJECT, Mode = BindingMode.TwoWay });
}
That's one way to achieve what you want, you could also use a converter too, or create a StaticResource in your Resources for the VM and bind to that as a source.