线程不会同时启动

这里有一个更新画布背景和元素的类，我想让背景与元素分开更新，因为元素更新需要相当多的时间，但是当我使用线程时，我没有注意到任何区别，当我暂停其中一个时，它们都被暂停了，所以我想两者都在同一个线程上，这是代码

namespace 
{
    class CanvasHandler
    {
        private Canvas _canvas;
        private Grid gird;
        private Brush _image;
        private UiHandler uiHandler;
        private Thread thread,thread1;
        public CanvasHandler(Canvas canvas, UiHandler uiHandler)
        {
            this._canvas = canvas;
            this.uiHandler = uiHandler;
            this.gird = this._canvas.Parent as Grid;
        }
        public void Update()
        {
            thread = new Thread(UpdateImage);
            thread.Start();
            thread1 = new Thread(UpdateCanvas);
            thread1.Start();

        }
        public void UpdateImage()
        {
            this._canvas.Dispatcher.BeginInvoke((Action)(() => this._canvas.Background = uiHandler.SourceCanvas.Background));
        }
        public void UpdateCanvas()
        {
            this._canvas.Dispatcher.BeginInvoke((Action)(UpdateCanvas_));

        }
        private void UpdateCanvas_()
        {
            Thread.Sleep(500);
            this._canvas.Children.Clear();
            foreach (UIElement child in uiHandler.SourceCanvas.Children)
            {
                var clone = Clone(child);
                this._canvas.Children.Add(clone);

            }
        }
        public T Clone<T>(T source)
        {
            string objXaml = XamlWriter.Save(source);
            var stringReader = new StringReader(objXaml);
            XmlReader xmlReader = XmlReader.Create(stringReader);
            var t = (T)XamlReader.Load(xmlReader);
            return t;
        }
    }
}