copia e incolla questa demo

codice:
<!doctype html>
<html lang="it">
<head>
  <meta charset="utf-8">
</head>

<body>  

    <form action="script1.php" id="form1">
        <fieldset>
            <input type="text" name="i1" value="v1" />
            <input type="text" name="i2" value="v2" />
            <input type="text" name="i3" value="v3" />
        </fieldset>
        <button type="submit">invia form 1</button>
    </form>

    <form action="script2.php" id="form2">
        <fieldset>
            <input type="text" name="i4" value="v4" />
            <input type="text" name="i5" value="v5" />
            <input type="text" name="i6" value="v6" />
        </fieldset>
        <button type="submit">invia form 2</button>
    </form>
</body>
		

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js"> </script>   
<script>
$(document).ready(function() {
    var form1 = $('#form1'),
        form2 = $('#form2');
        
    form2.bind('submit', function(evt) {
        
        var s1 = form1.serialize(),        
            s2 = form2.serialize(),
            serializedData = [s1, '&', s2].join('');
            
        $.ajax({
            method  : 'post',
            url     : form2.attr('action'),
            data    : serializedData,              
        })  
        
        evt.preventDefault();   
    });   
});
</script>
Ciao