Non avendo conoscenze di jquery, ajax etc. ho preso uno scriptino e adattato al form per vedere l'output di serialize().
codice:
$(document).ready(function(){             $("#myform2").submit(function(){
                 var querystring = $(this).serialize();
                 $('#result').html(querystring);
                 return false;
                 });
        });

Il risultato è quello che dovrebbe essere, cioè "comune=venezia" esattamente come POST mi da con print_r(): Array ( [comune] => venezia)

é vero che ho sostituito lo script con un altro, quindi non so se va bene lo stesso
Comunque questa è la pagina semplificata con un unico form con cui sto facendo le prove e in ogni caso mi da il problema:

codice:
<?phprequire_once "library/config.php";
?>
<!DOCTYPE html>
<html lang="it">
	<head>
		<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
		<meta charset="utf-8" />
		
<link href="http://netdna.bootstrapcdn.com/bootstrap/3.1.0/css/bootstrap.min.css" rel="stylesheet">


<script type="text/javascript" src="http://code.jquery.com/jquery-1.7.1.min.js"></script>
	<script type="text/javascript" src="http://ajax.microsoft.com/ajax/jquery.validate/1.7/jquery.validate.min.js"></script>
    <script type="text/javascript">


$(document).ready(function(){
		$(".formValidation").validate({
			debug: false,
			rules: {
				name: "required",
				email: {
					required: true,
					email: true
				}
			},
			messages: {
				name: "Please let us know who you are.",
				email: "A valid email will help us get in touch with you.",
			},
			submitHandler: function(form) {
				// do other stuff for a valid form
				$('.formValidation').on('submit', function () {
    $.post('process.php', $(this).serialize(), function(data) {
        $('#results').html(data);
       });
})
			}
		});
	}); 
	</script>


</head>
	<body>




		<div class="main-container" id="main-container">
			<div class="main-content">
				<div class="page-content">
				  <div class="page-content-area">
						<div class="page-header">
							<h1>
							<br >
							</h1>
						</div>


						<div class="row">
							<div class="col-xs-12">
								


<form id="frmAddProduct" class="formValidation" action="processProduct.php?action=addProduct" method="post" enctype="multipart/form-data" name="frmAddProduct"></form>
<form id="myform" class="formValidation" name="myform" action="" method="post"></form>
<form id="myform2" class="formValidation" name="myform2" action="" method="post"></form>
<table class="table table-striped">


 <tr> 
 <td>
<div class="col-xs-12 col-sm-4">
											<div class="widget-box">
												


												<div class="widget-body">
													<div class="widget-main">
														<div>
															<label for="form-field-select-1">Comune</label>
                                <select name="comune" class="form-control" id="form-field-select-1" form="frmAddProduct">
                                  <option value="">Seleziona...</option>
													<?php 		
                             $comune = "SELECT * FROM tbl_comune ORDER BY nome_comune ASC";
                                 $result_comune = dbQuery($comune); 
                                   if (dbNumRows($result_comune) > 0) {
                                      while($row_comune = dbFetchAssoc($result_comune)) {
	                                      extract($row_comune);
                                     ?>   
																
																<option value="<?php echo $id_comune; ?>"><?php echo $nome_comune; ?></option>
																 <?php
                                   }
                                  } else {
                                ?>
                                  <option value="">Non ci sono dati</option>
															   <?php
                                 }
                                 ?>
                              </select>
														</div>


													<hr>	
											<div class="widget-body">
												<div class="widget-main">
													<div> 
                       	     
           <input type="text" name="comune" id="comune" value="" placeholder="Aggiungi Comune" form="myform2">  
	            <input type="submit" name="submit" value="Submit" class="btn btn-sm btn-success" form="myform2"> 
              <p id="result"></p>
       <div id="results"></div>
												</div>
											</div>
										</div>
										 </div>
											 </div>
										     </div>
                           </div>




</td>							  
  </tr>
    </table>
 
               
              </div><!-- /.col -->
						</div><!-- /.row -->
					</div><!-- /.page-content-area -->
				</div><!-- /.page-content -->
			</div><!-- /.main-content -->
     </div><!-- /.main-container -->
  
	</body>
</html>
il file process.php non fa altro che eseguire la query:

codice:
 foreach( $_POST as $key => $value ) {     
$sql   = "INSERT INTO tbl_".$key."(nome_".$key.") VALUES ('$value')";
$result = dbQuery($sql); 

}