代码段1：只能打开一个save ？？？请问是怎么回事？？

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 f = new Form2();
            f.Show();
            Form2 f1 = new Form2();
            f1.Show();
        }
    }

public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
            this.FormClosing += new FormClosingEventHandler(Form2_FormClosing);
            Thread td = new Thread(DO);
            td.Name = "from2";
            td.Start();
        }

        private void DO()
        {
            Thread.Sleep(2000);
            this.Invoke(new Action(() =>
            {
                this.Close();
            }));
        }

        void Form2_FormClosing(object sender, FormClosingEventArgs e)
        {
            System.Windows.Forms.SaveFileDialog sd = new SaveFileDialog();
            sd.ShowDialog();
        }
    }