把两个EXCEL都导入DataTable中，然后进行合并，再输出新的Excel，或覆盖“Exce2l.xls”。

#region 导入Excel文件
public static DataTable ImportExcel(string filePathName)
{
    string strConn = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + HttpContext.Current.Server.MapPath(filePathName) + ";Extended Properties=Excel 8.0";
    //链接Excel
    OleDbConnection cnnxls = new OleDbConnection(strConn);
    //读取Excel里面有 表Sheet1
    string abc = filePathName.Substring(filePathName.LastIndexOf("/") + 1);
    OleDbDataAdapter oda = new OleDbDataAdapter("select * from [" + abc + "$]", cnnxls);
    DataSet ds = new DataSet();
    //将Excel里面有表内容装载到内存表中！
    oda.Fill(ds);
    return ds.Tables[0];
}
#endregion

#region 根据传入的DataTable并导出为Excel文件(无HTML装饰)
/// <summary>
/// 根据传入的DataTable并导出为Excel文件
/// </summary>
/// <param name="dt">传入的DataTable</param>
/// <param name="fileName">文件名</param>
public static void ExportExcel(DataTable dt, string fileName)
{
    if (dt == null || dt.Rows.Count == 0) { return; }
    HttpContext.Current.Response.Clear();
    //Encoding pageEncode = Encoding.GetEncoding(PageEncode);
    HttpContext.Current.Response.Charset = "gb2312";
    //Response.ContentType = "application/vnd-excel";//"application/vnd.ms-excel";
    //Response.ContentType = "application/x-octet-stream";//"application/vnd.ms-excel";
    HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";//"text/csv";
    HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment;filename=" + fileName + ".xls");
    System.IO.StringWriter oSW = new System.IO.StringWriter();
    HtmlTextWriter oHW = new HtmlTextWriter(oSW);
    DataGrid dg = new DataGrid();
    dg.DataSource = dt;
    dg.DataBind();
    dg.RenderControl(oHW);
    HttpContext.Current.Response.Write(oSW.ToString());
    HttpContext.Current.Response.Flush();
    HttpContext.Current.Response.Close();
}
#endregion