cioa prova cosi

codice:
<select name="peso" onchange="if (this.value=='compreso'){document.getElementById('novo_textbox').innerHTML='Text1: <input type=\'text\' name=\'TEXT1\' /> Text2: <input type=\'text\' name=\'TEXT2\' />'}else{document.getElementById('novo_textbox').innerHTML='Text1: <input type=\'text\' name=\'TEXT1\' />'}">
<option value="maggiore">maggiore</option>
<option value="minore">minore</option>
<option value="compreso">compreso</option>
</select>
<span id="novo_textbox">Text1: <input type="text" name="TEXT1" /></span>