Merging multiple reports and assigning it to ReportSource of XRSubReport

Question

This code gives me proper output. I can see the subReport inside my vpReport. But,

    private void createReport(List<DataSet> dataList> 
    {
      ENG.Report vpReport = new UI.ENG.Report();
    
      ENG.SubReport subReport = new ENG.SubReport();
      subReport.DataSource = dataList[0];
      (vpReport.FindControl("xrSubReport1", true) as XRSubreport).ReportSource = subReport;
      vpReport.ShowPreview();
    }

This code just gives me blank in place of xrSubReport1. I don't understand why. I tried setting margin lesser than vpReport too. I tried adding pages directly to vpReport. Nothing works. How do I achieve this. I need to merge all the subReport i have, and add that to my main report vpReport.

private void createReport(List<DataSet dataList) 
{
  ENG.Report vpReport = new UI.ENG.Report();
  XtraReport mergedReport = new XtraReport();
  foreach (DataSet ds in dataList)
  {
     ENG.SubReport subReport = new ENG.SubReport();
     subReport.DataSource = ds;
     subReport.CreateDocument();
     mergedReport.Pages.AddRange(subReport.Pages);
  }
  (vpreport.FindControl("xrSubReport1", true) as XRSubreport).ReportSource = mergedReport;
  vpReport.ShowPreview();
}

Please guide me