PdfDocument pdfDoc = new PdfDocument();

string strDeviceInfo = "<DeviceInfo><HumanReadablePDF>True</HumanReadablePDF></DeviceInfo>";
string strMimeType = "";
string strEncoding = "";
string strFileNameExtension = "";
string[] streams;
Warning[] warnings;

// First local report generated with MS Reporting
LocalReport report1 = new LocalReport();
report1.DataSources.Add(new ReportDataSource("MyDS", myDataSource1));
report1.ReportEmbeddedResource = "myreport.rdlc";
byte[] pdfContent1 = report1.Render("PDF", strDeviceInfo, strMimeType, strEncoding, strFileNameExtension, streams, warnings);
MemoryStream ms1 = new MemoryStream(pdfContent1);
PdfDocument tmpPdfDoc1 = PdfReader.Open(ms1, PdfDocumentOpenMode.Import);
for (int ind = 0; ind < tmpPdfDoc1.PageCount; ind++) {
    pdfDoc.AddPage(tmpPdfDoc1.Pages(ind));
}

// Second local report generated with MS Reporting
LocalReport report2 = new LocalReport();
report2.DataSources.Add(new ReportDataSource("MyDS", myDataSource2));
report2.ReportEmbeddedResource = "myreport.rdlc";
byte[] pdfContent2 = report2.Render("PDF", strDeviceInfo, strMimeType, strEncoding, strFileNameExtension, streams, warnings);
MemoryStream ms2 = new MemoryStream(pdfContent2);
PdfDocument tmpPdfDoc2 = PdfReader.Open(ms2, PdfDocumentOpenMode.Import);
for (int ind = 0; ind < tmpPdfDoc2.PageCount; ind++) {
    pdfDoc.AddPage(tmpPdfDoc2.Pages(ind));
}

// Save the merged pdf
strFile = System.IO.Path.GetTempFileName.Replace(".tmp", ".pdf");
pdfDoc.Save(strFile);

// Open the merged pdf
System.Diagnostics.Process proceso = new System.Diagnostics.Process();
proceso.StartInfo.FileName = strFile;
proceso.Start();