You can put each of your tables in an iTextSharp.text.Paragraph and use the Paragraph object's SpacingAfter property to create your gap.
Like this test method:
private static void DemoTableSpacing() {
    using (FileStream fs = new FileStream("SpacingTest.pdf", FileMode.Create)) {
        Document doc = new Document();
        PdfWriter.GetInstance(doc, fs);
        doc.Open();
        Paragraph paragraphTable1 = new Paragraph();
        paragraphTable1.SpacingAfter = 15f;
        PdfPTable table = new PdfPTable(3);
        PdfPCell cell = new PdfPCell(new Phrase("This is table 1"));
        cell.Colspan = 3;
        cell.HorizontalAlignment = 1;
        table.AddCell(cell);
        table.AddCell("Col 1 Row 1");
        table.AddCell("Col 2 Row 1");
        table.AddCell("Col 3 Row 1");
        //table.AddCell("Col 1 Row 2");
        //table.AddCell("Col 2 Row 2");
        //table.AddCell("Col 3 Row 2");
        paragraphTable1.Add(table);
        doc.Add(paragraphTable1);
        Paragraph paragraphTable2 = new Paragraph();
        paragraphTable2.SpacingAfter = 10f;
        table = new PdfPTable(3);
        cell = new PdfPCell(new Phrase("This is table 2"));
        cell.Colspan = 3;
        cell.HorizontalAlignment = 1;
        table.AddCell(cell);
        table.AddCell("Col 1 Row 1");
        table.AddCell("Col 2 Row 1");
        table.AddCell("Col 3 Row 1");
        table.AddCell("Col 1 Row 2");
        table.AddCell("Col 2 Row 2");
        table.AddCell("Col 3 Row 2");
        paragraphTable2.Add(table);
        doc.Add(paragraphTable2);
        doc.Close();
    }
}
This should show what you can do.  Try adding and removing rows in that first table; you'll see that the space between the two tables is always there and doesn't change.