Use
StringBuilder class[
^] to create string you want to save in single cell.
StringBuilder sb = new StringBuilder();
PdfPCell subCell1 = new PdfPCell();
for (...)
{
sb.AppendLine(String.Concat(dtResult.Rows[i][5].ToString(),":-",dtResult.Rows[i][6].ToString()));
}
subCell1.Phrase = new Phrase(sb.ToString(), FontFactory.GetFont("verdana", 8, BaseColor.BLACK));
subTable1.AddCell(subCell1);