To create a MultiChart PDF file, simply add Charts to the list. The contents of the list at export (save) time is the contents that will be exported.
//No need to add the calling Chart, it is added automatically.
//Add any other Charts you require to be exported.
tChart1.Export.Image.PDF.MultiChart.Charts.Add(tChart2.Chart);
//_or_ optionally replace the calling Chart if not required:   
tChart1.Export.Image.PDF.MultiChart.Charts[0] = tChart2.Chart;
//_or_ remove the calling Chart if not required:
tChart1.Export.Image.PDF.MultiChart.Charts.RemoveAt(0);
tChart1.Export.Image.PDF.Save(@"c:\temp\MyTeeCharts.pdf");
public MultiPDFChart MultiChart {get; set;}

See Also

PDFFormat Class | PDFFormat Members | Steema.TeeChart.Export Namespace