If you want union of two data table in asp.net then follow this.....

        public DataTable Union(DataTable First, DataTable Second)
        {

            //Result table
            DataTable table = new DataTable("Union");

            //Build new columns
            DataColumn[] newcolumns = new DataColumn[First.Columns.Count];

            for (int i = 0; i < First.Columns.Count; i++)
            {
                newcolumns[i] = new DataColumn(
                First.Columns[i].ColumnName, First.Columns[i].DataType);
            }

            table.Columns.AddRange(newcolumns);
            table.BeginLoadData();

            foreach (DataRow row in First.Rows)
            {
                table.LoadDataRow(row.ItemArray, true);
            }

            foreach (DataRow row in Second.Rows)
            {
                table.LoadDataRow(row.ItemArray, true);
            }

            table.EndLoadData();
            return table;
        }