<html>
<head>
<title></title>
</head>
<body>
<SELECT ID="Select1" NAME="Select1" onclick="javascript
:Cliccato();">
<OPTION value="">-</OPTION>
<option value="x">test1</option>
<option value="x">test2</option>
<option value="x">test3</option>
</SELECT>
<SELECT ID="Select2" NAME="Select1"">
<OPTION value="">-</OPTION>
<option value="x">test4</option>
<option value="x">test5</option>
<option value="x">test6</option>
</SELECT>
<script language=javascript>
function Cliccato(){
if (document.getElementById('Select1').value !=''){ // Se selezionato qualcosa
document.getElementById('Select2').disabled = true; // Disabilito
} else {
document.getElementById('Select2').disabled = false;// se no riabilito
}
}
</script>
</body>
</html>