Avevo fatto degli errori scrivendolo al volo (senza testarlo) ora va bene:
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>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Untitled Document</title>
<script language="javascript" type="text/javascript">
function showhide(iDiv){
var el = document.getElementsByTagName("div")
for(var i = 0; i<el.length; i++){ 
if (el[i].getAttribute("id") == iDiv && el[i].style.display=='none'){
el[i].style.display = "block";
} else {
el[i].style.display = "none";
}
}
}
</script>
</head>

<body>
<div style="border:2px solid #000080; padding:5px; display: none;  background:#FFFFFF;" id="pippo">TESTO TESTO TESTO


chiudi</p>
</div>


apri il div</p>

<div style="border:2px solid #000080; padding:5px; display: none; background:#FFFFFF;" id="pluto">TESTO TESTO TESTO


chiudi</p>
</div>


apri il div</p>
</body>
</html>