Prova cosi:
codice:
function onsubmitform(tasto)
{
  if(tasto == 'Insert')
  {
   document.myform.action ="insert.html";
document.myform.submit();
  }
  else
 {
    if(tasto== 'Update')
    {
      document.myform.action ="update.html";
document.myform.submit();
    }
 }
  return true;
}
</script>

<form name="myform" onsubmit="return false;">
<input type="button" name="operation" onclick="onsubmitform('Insert')" value="insert" />
<input type="button" name="operation" onclick="onsubmitform('Update')" value="update" />
</form>