Try this solution I hope it may help you
System.Data.SqlClient.SqlConnection con = new System.Data.SqlClient.SqlConnection(@"Connection String");
System.Data.SqlClient.SqlDataAdapter da = new System.Data.SqlClient.System.Data.DataTable dt = new System.Data.protected void Page_Load(object sender, EventArgs e)
{
con.Open();
da = new System.Data.SqlClient.SqlDataAdapter
("SELECT Name FROM Namedata UNION SELECT City FROM Citydata", con);
da.Fill(dt);
foreach(System.Data.DataRow D in dt.Rows)
{
DropDownList1.Items.Add(D[0].ToString());
}
con.Close();
}