prova così:

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

<script language="JavaScript">

function Riempi(f) {
 var v1=f.nomecampo1.value;
 var v2=f.nomecampo2.value;
 var k=0;
 if (v1=="" && v2=="") {
  alert("Compilare entrambi i campi!");
  f.nomecampo1.focus();
  f.nomeselect.options.length=0;
  f.nomeselect.disabled=true;
  return;
 }
 for (var i=1; i<3; i++)
  if (f.elements["nomecampo"+i].value!="") {
    f.nomeselect.options[k]=new Option(f.elements["nomecampo"+i].value, f.elements["nomecampo"+i].name);
    k++;
  }
 f.nomeselect.disabled=false;
 f.nomeselect.options.length=k;
}


</script>

</head>

<body>
<form name="f">
  


    <input type="text" name="nomecampo1">
  </p>
  

 
    <input type="text" name="nomecampo2">
    <input type="button" name="Submit" value="Pulsante" onClick="Riempi(f);">
  </p>
  

 </p><select name="nomeselect" disabled></select>
</form>
</body>
</html>
ciao