var bytes = _service.MergeExcelWorksheets();

 if (bytes == null)
     return null;

 var fileStream = new MemoryStream(bytes);

 var contentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
 var fileDownloadName = string.Format("test{0}.xlsx", DateTime.Now.ToString("yyyyMMdd"));

 fileStream.Position = 0;
 var fsr = new FileStreamResult(fileStream, contentType);
 fsr.FileDownloadName = fileDownloadName;

 var bytes = _service.MergeExcelWorksheets();

 if (bytes == null)
     return null;

 var fileStream = new MemoryStream(bytes);

 var contentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
 var fileDownloadName = string.Format("test{0}.xlsx", DateTime.Now.ToString("yyyyMMdd"));

 fileStream.Position = 0;
 var fsr = new FileStreamResult(fileStream, contentType);
 fsr.FileDownloadName = fileDownloadName;

        private byte[] MergeExcelWorksheets(byte[] array1, byte[] array2)
        {
            using (MemoryStream ms1 = new MemoryStream(array1))
            using (MemoryStream ms2 = new MemoryStream(array2))
            using (ExcelPackage package1 = new ExcelPackage(ms1))
            using (ExcelPackage package2 = new ExcelPackage(ms2))
            {
                var workbook1 = package1.Workbook;
                var workbook2 = package2.Workbook;

                // Iterate through sheets in workbook2
                foreach (var sourceSheet in workbook2.Worksheets)
                {
                    if (!workbook1.Worksheets.Any(sheet => sheet.Name == sourceSheet.Name))
                    {
                        // Clone the sheet and add it to workbook1
                        var clonedSheet = workbook1.Worksheets.Add(sourceSheet.Name, sourceSheet);
                    }
                }

                return ms1.ToArray();
            }
        }

        private byte[] MergeExcelWorksheets(byte[] array1, byte[] array2)
        {
            using (MemoryStream ms1 = new MemoryStream(array1))
            using (MemoryStream ms2 = new MemoryStream(array2))
            using (ExcelPackage package1 = new ExcelPackage(ms1))
            using (ExcelPackage package2 = new ExcelPackage(ms2))
            {
                var workbook1 = package1.Workbook;
                var workbook2 = package2.Workbook;

                // Iterate through sheets in workbook2
                foreach (var sourceSheet in workbook2.Worksheets)
                {
                    if (!workbook1.Worksheets.Any(sheet => sheet.Name == sourceSheet.Name))
                    {
                        // Clone the sheet and add it to workbook1
                        var clonedSheet = workbook1.Worksheets.Add(sourceSheet.Name, sourceSheet);
                    }
                }

                return ms1.ToArray();
            }
        }