Cosi

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function pippo(aCB) {
 prefix = aCB.id.substr(0,3);
 if (prefix+'S' == aCB.id) {
  document.getElementById(prefix+'N').checked = ! aCB.checked;
 } else {
  document.getElementById(prefix+'S').checked = ! aCB.checked;
 }
}
//-->
</script>

</head>
<body>
<input type="checkbox" id="cb1S" onclick="pippo(this)">Si

<input type="checkbox" id="cb1N" onclick="pippo(this)">No

</body>
</html>