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);
}