<Window x:Class="DualMonitors.Views.WinLeft"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"

        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

        xmlns:local="clr-namespace:DualMonitors.Views"

        mc:Ignorable="d" WindowState="Maximized"  WindowStartupLocation="CenterScreen"

        Title="WinLeft" Height="300" Width="300">

    <Grid Background="Blue">

        <Grid>

            <Grid.RowDefinitions>

                <RowDefinition/>

                <RowDefinition/>

            </Grid.RowDefinitions>

            <TextBlock Text="Left" FontSize="50" VerticalAlignment="Center" HorizontalAlignment="Center"/>

            <Button Grid.Row="2" Command="{Binding LeftCmd}" Content="Click Left" FontSize="50"/>

        </Grid>

    </Grid>

</Window>

<Window x:Class="DualMonitors.Views.WinRight"

        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"

        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"

        xmlns:local="clr-namespace:DualMonitors.Views"

        mc:Ignorable="d" WindowState="Maximized" WindowStartupLocation="CenterScreen"

        Title="WinRight">

    <Grid Background="Yellow">

        <Grid>

            <Grid.RowDefinitions>

                <RowDefinition/>

                <RowDefinition/>

            </Grid.RowDefinitions>

            <TextBlock Text="Right" FontSize="50" VerticalAlignment="Center" HorizontalAlignment="Center"/>

            <Button Grid.Row="2" Command="{Binding RightCmd}" Content="Click Right" FontSize="50"/>

        </Grid>

    </Grid>

</Window>

using DualMonitors.ViewModel;

using System.Windows;

namespace DualMonitors.Views

{

    /// <summary>

    /// Interaction logic for WinRight.xaml

    /// </summary>

    public partial class WinRight : Window

    {

        public WinRight()

        {

            InitializeComponent();

            DualVM BDualVM = DualVM.GetDualVM();

            this.DataContext = BDualVM;

        }

    }

}

using Prism.Commands;

using System.Windows;

namespace DualMonitors.ViewModel

{

    public class DualVM

    {

        private static DualVM DualVMInstance;

        public static DualVM GetDualVM()

        {

            object objLock = new object();

            lock(objLock)

            {

                if(DualVMInstance==null)

                {

                    DualVMInstance = new DualVM();

                }

            }

            return DualVMInstance;

        }

        private DualVM()

        {

            //MessageBox.Show("Dual Monitor");

        }

        private DelegateCommand leftCmdValue;

        public DelegateCommand LeftCmd

        {

            get

            {

                if(leftCmdValue==null)

                {

                    leftCmdValue = new DelegateCommand(LeftCmdExecuted);

                }

                return leftCmdValue;

            }

        }

        private void LeftCmdExecuted()

        {

            MessageBox.Show("Left Monitor");

        }

        private DelegateCommand rightCmdValue;

        public DelegateCommand RightCmd

        {

            get

            {

                if(rightCmdValue==null)

                {

                    rightCmdValue = new DelegateCommand(RightCmdExecuted);

                }

                return rightCmdValue;

            }

        }

        private void RightCmdExecuted()

        {

            MessageBox.Show("Right Monitor");

        }

    }

}