Animating both values at the same time

        private void ResizeWindow(double width, double height)
        {
            Duration duration = TimeSpan.FromSeconds(2);

            DoubleAnimation widthAnimation = new DoubleAnimation();
            widthAnimation.To = width;
            widthAnimation.Duration = duration;

            DoubleAnimation heightAnimation = new DoubleAnimation();
            heightAnimation.To = height;
            heightAnimation.Duration = duration;

            Storyboard storyboard = new Storyboard();
            storyboard.Children.Add(widthAnimation);
            storyboard.Children.Add(heightAnimation);
            Storyboard.SetTarget(widthAnimation, this);
            Storyboard.SetTarget(heightAnimation, this);
            Storyboard.SetTargetProperty(widthAnimation, new PropertyPath(Window.WidthProperty));
            Storyboard.SetTargetProperty(heightAnimation, new PropertyPath(Window.HeightProperty));

            storyboard.Begin();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            ResizeWindow(640, 360);
        }