C#: Export multiple datagrids into multiple sheets to Excel with formatting

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C#

MS-Excel

DataGrid

Response.ClearContent();
Response.Buffer = true;
System.IO.StringWriter sw = new System.IO.StringWriter();
HtmlTextWriter htw = new HtmlTextWriter(sw);
Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", "Summary Report " + _startDate.ToString("m") + " to " + _endDate.ToString("m") + ".xls"));
Response.ContentType = "application/ms-excel";
SummaryGrid.HeaderStyle.BackColor = System.Drawing.Color.LightGray;
SummaryGrid.GridLines = GridLines.Both;
SummaryGrid.RenderControl(htw);
this.ClearControls(TimeEntryGrid);
TimeEntryGrid.AllowPaging = true;
TimeEntryGrid.Columns[7].ItemStyle.Width = 450;
TimeEntryGrid.Columns[7].ItemStyle.HorizontalAlign = HorizontalAlign.Left;
TimeEntryGrid.HeaderStyle.BackColor = System.Drawing.Color.LightGray;
TimeEntryGrid.GridLines = GridLines.Both;
TimeEntryGrid.RenderControl(htw);
Response.Write(sw.ToString());
Response.End()

SummaryGrid and TimeEntryGrid are two Datagrids. I want to separate them into two sheets WITHOUT using any references or libraries while preserving the format of datagrids. How do I split them into two sheets in excel? Or other there any other ways to format the datagrids so that they will not be joined together vertically in one sheet?