gistfile1.cs

using System.Threading;
using System.Windows;
using System.Windows.Threading;

namespace BorderTest
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        public delegate void MyDelegate();

        public void dispatchP1()
        {
            player1.Dispatcher.VerifyAccess();
            player1.Dispatcher.Invoke(new MyDelegate(p1SetContent));
        }

        public void p1SetContent()
        {
            player1.Content = "WORKS!";
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            var thread = new Thread(() => new SecondMainWindow().dispatchP1());
            thread.SetApartmentState(ApartmentState.STA);
            thread.Start();
        }
    }


    public class SecondMainWindow : MainWindow
    {
    }
}