prova questo esempio:

<html>
<head>
<title>Untitled</title>
<script>
function visualizza() {
if (document.getElementById("s0").value=="1")
document.getElementById("s1").style.visibility="vi sible";
else
document.getElementById("s1").style.visibility="hi dden";
if (document.getElementById("s0").value=="2")
document.getElementById("s2").style.visibility="vi sible";
else
document.getElementById("s2").style.visibility="hi dden";
}
</script>
</head>

<body>
<select name="s0" id="s0" onchange="visualizza();">
<option value="0"></option>
<option value="1">1</option>
<option value="2">2</option>
</select>
<select name="s1" id="s1" style="visibility:hidden">
<option value="1">1</option>
<option value="2">2</option>
</select>
<select name="s2" id="s2" style="visibility:hidden">
<option value="1">1</option>
<option value="2" selected>2</option>
</select>
</body>
</html>