How to get a certain UI control from two classes

I'm developing a UWP application with two window pages on two separate screens.

It's include:

• Mainpage.xaml
• Mainpage.xaml.cs
• Page2.xaml
• Page2.xaml.cs

My scenario:

1. get the mouse position(X) while moving on the secondary screen (Page2).
2. write it in the text block on the first screen (MainPage), Changing at the same time.

In MainPage.xaml

<Page
    x:Class="LineDraw.MainPage"
    xmlns:local="using:LineDraw"
    ...
    ..

    <Grid>

    <TextBlock Name="textblock" Text="N/A" HorizontalAlignment="Stretch" VerticalAlignment="Top" Margin="100" />

    </Grid>
</Page>

In MainPage.xaml.cs

 public sealed partial class MainPage : Page
    {   
        public double Xpage2;

        public MainPage()
        {
            this.InitializeComponent();
            newpage();   
        }

        private async void newpage()
        { 
            int NewWindowid = 0;
            int Windowid = ApplicationView.GetForCurrentView().Id;
            CoreApplicationView newView = CoreApplication.CreateNewView();
            await newView.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
            {
                Frame newframe = new Frame();
                newframe.Navigate(typeof(Page2), this); // this means existing MainPage object.
                Window.Current.Content = newframe;
                Window.Current.Activate();
                NewWindowid = ApplicationView.GetForCurrentView().Id;
            });

             bool available = ProjectionManager.ProjectionDisplayAvailable;

             ProjectionManager.ProjectionDisplayAvailableChanged += (s, e) =>
             {
               available = ProjectionManager.ProjectionDisplayAvailable;
             };

            await ProjectionManager.StartProjectingAsync(NewWindowid, Windowid);
        }

        public void copyX(double Xpage2)
        {
            textblock.Text = $"X = {Xpage2}";  
        }
}

In Page2.xaml.cs

 public sealed partial class Page2 : Page
    {      
        MainPage mainpage;
        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            mainpage = e.Parameter as MainPage;
            base.OnNavigatedTo(e);
        }

        public Page2()
        {
            this.InitializeComponent();    
            Window.Current.CoreWindow.PointerMoved += CoreWindow_PointerMoved;    
        }

        public void CoreWindow_PointerMoved(CoreWindow sender, PointerEventArgs args)
        {
            Point ptr = args.CurrentPoint.Position;
            double Xpage2 = ptr.X;
            mainpage.copyX(Xpage2);

        }

I did the code above but the result is the following error:

System.Exception
  HResult=0x8001010E
  Message=The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))
  Source=<Cannot evaluate the exception source>
  StackTrace:
<Cannot evaluate the exception stack trace>

I want to get the textblock(UI control) in main.xaml from Page2.xaml.cs. I need a solution , How can I do that?

You're trying to run a method (mainpage.copyX()) that has been running in another context. I don't know why you created another view, if you can just run this code on the MainView of CoreApplication. In MainPage.xmal.cs:

await CoreApplication.MainView.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
     Frame newframe = new Frame();
     newframe.Navigate(typeof(Page2), this);
     Window.Current.Content = newframe;
     Window.Current.Activate();
     NewWindowid = ApplicationView.GetForCurrentView().Id;
});

await CoreApplication.MainView.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
     Point ptr = args.CurrentPoint.Position;
     double Xpage2 = ptr.X;
     mainpage.copyX(Xpage2);
});

If you create another view, you need somehow share this resource in order for everyone execute on the same context.