Sync multiple MediaItems
private void slider1_DragCompleted(object sender, System.Windows.Controls.Primitives.DragCompletedEventArgs e)
    {

        videos[0].Position = TimeSpan.FromSeconds(slider1.Value);

        foreach (MediaElement elem in videos)
        {
            elem.Position = videos[0].Position;
            elem.Play();
        }

private void slider1_DragCompleted(object sender,
    System.Windows.Controls.Primitives.DragCompletedEventArgs e)
{
    var realPosition = TimeSpan.FromSeconds(slider1.Value);

    foreach (var elem in videos)
    {
        elem.Position = realPosition;
    }

    foreach (var elem in videos)
    {
        elem.Play();
    }