How to set Wpf Prism Region ZIndex

Question

If there are two prism regions defined in a shell, which one is on top is determined by how these are layed out in the shell one below the other.

<Grid Grid.Row="1" x:Name="_redGrid" >
    <ContentControl prism:RegionManager.RegionName="RedRegion" />
</Grid>
<Grid Grid.Row="1" x:Name="_greenGrid">
    <ContentControl prism:RegionManager.RegionName="GreenRegion" />
</Grid>

For a simple example, you can take a look at this. It has got a red view and a green view. When you run, it will look like the following.

Is there a way to change this dynamically? Set the zindex of the prism region at run time?

I have extended the above example with a button, objective here is to click the button and change the top to bottom. You can see this example here.

I could do this in the backend file with brute force as follows.

private void btn1_Click(object sender, RoutedEventArgs e)
{
    _mainGrid.Children.Remove(_redGrid);
    _mainGrid.Children.Remove(_greenGrid);

    if (_toggleRegion)
    {
        _mainGrid.Children.Add(_redGrid);
        _mainGrid.Children.Add(_greenGrid);
    }
    else
    {
        _mainGrid.Children.Add(_greenGrid);
        _mainGrid.Children.Add(_redGrid);
    }
    _toggleRegion = !_toggleRegion;
}

Click the button, and now the red is on top.

So whats the best way to do this? Is there a feature in Prism which can accomplish this?