codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Pagina senza titolo</title>

    <script language="javascript" type="text/javascript">
// <!CDATA[
function funzione()
{
    var risultato = 0;
    for(var i = 1; i <= 2; i++)
    {
        var v = document.getElementById("vote" + i).value;
        risultato += parseInt(v, 10);
    }
    if(!isNaN(risultato))
        document.getElementById("risultato").value = risultato;
    else
        document.getElementById("risultato").value = "Completa la votazione";
}


// ]]>
    </script>
</head>
<body>
    <select id="vote1" onchange="funzione();">
        <option value="" selected="selected">Vote</option>
        <option value="1">1</option>
        <option value="2">2</option>
        <option value="3">3</option>
        <option value="4">4</option>
        <option value="5">5</option>
        <option value="6">6</option>
        <option value="7">7</option>
        <option value="8">8</option>
        <option value="9">9</option>
        <option value="10">10</option>
    </select>
    <select id="vote2" onchange="funzione();">
        <option value="" selected="selected">Vote</option>
        <option value="1">1</option>
        <option value="2">2</option>
        <option value="3">3</option>
        <option value="4">4</option>
        <option value="5">5</option>
        <option value="6">6</option>
        <option value="7">7</option>
        <option value="8">8</option>
        <option value="9">9</option>
        <option value="10">10</option>
    </select>
    
    <input id="risultato" type="text" size="5" maxlength="5" disabled="disabled" value="qui dovrebbe comparire la somma!"/>
</body>
</html>