C# Response.ContentType Creating multiple tabs in Excel

Question

I'm generating an Excel report in C# and need to add a second tab to the Excel file that will contain reference data. How do I achieve this without using ClosedXML ?

Response.AddHeader("content-disposition", "attachment; filename=" + fname);
Response.ContentType = "application/vnd.ms-excel";
var writer = new System.IO.StringWriter();
var htmlTextWriter = new HtmlTextWriter(writer);
dataGrid.RenderControl(htmlTextWriter);
Response.Write(writer.ToString());
Response.End();