C# Combobox ต้องการแสดง DisplayMember 2 ค่าทำยังไงครับ

        private void button1_Click(object sender, EventArgs e)
        {
            string connetionString = null;
            SqlConnection connection;
            SqlCommand command;
            SqlDataAdapter adapter = new SqlDataAdapter();
            DataSet ds = new DataSet();
            int i = 0;
            string sql = null;

            connetionString = "Data Source=ServerName;Initial Catalog=databasename;User ID=userid;Password=yourpassword";
            sql = "select au_id,au_lname from authors";

            connection = new SqlConnection(connetionString);

            try
            {
                connection.Open();
                command = new SqlCommand(sql, connection);
                adapter.SelectCommand = command;
                adapter.Fill(ds);
                adapter.Dispose();
                command.Dispose();
                connection.Close();

                comboBox1.DataSource = ds.Tables[0];
                comboBox1.ValueMember = "au_id";
                comboBox1.DisplayMember = "au_lname";

            }
            catch (Exception ex)
            {
                MessageBox.Show("Can not open connection ! ");
            }
        }