How do I open two windows on launch in WPF?

I feel like I am missing something very fundamental here, but I can't seem to find an answer.

When my app launches, I want a second window based on existing xaml to open in addition to MainWindow.xaml. I've found a lot about using code-behind to create a NEW window, but I want to open a window that is predefined in another xaml file.

Both are using MahApps and are defined as

<Controls:MetroWindow x:Class=...
  ...
</Controls:MetroWindow>

The second window is called ControlWindow.xaml and sits in the root with MainWindow.xaml

Thank you

EDIT:

When trying to create and show the window in the App_Startup event in app.xaml.cs, even though the window inherits from the same class as MainWindow.xaml, it does not have the Show() method available.

MainWindow.xaml.cs

using MahApps.Metro.Controls;

namespace RollCallDisplayDemo
{
    public partial class MainWindow : MetroWindow
    {        
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

ControlWindow.xaml.cs

using MahApps.Metro.Controls;

namespace RollCallDisplayDemo
{
    public partial class ControlWindow : MetroWindow
    {
        public ControlWindow()
        {
            InitializeComponent();
        }
    }
}

App.xaml.cs

using System.Windows;
using GalaSoft.MvvmLight.Threading;

namespace RollCallDisplayDemo
{
    public partial class App : Application
    {
        void App_Startup(object sender, StartupEventArgs e)
        {
            MainWindow NewWindowA = new MainWindow();
            ControlWindow NewWindowB = new ControlWindow();
        }

        static App()
        {
            DispatcherHelper.Initialize();
        }
    }
}

NewWindowA acts as you would expect and allows a new instance to be created and shown. NewWindowB only has the InitializeComponent method available, nothing else which should inherited from the MetroWindow class.