Réponse acceptée !
<html>
<head>
<script language="javascript">
function changeFormAction(destination)
{
document.form1.action = destination;
document.form1.submit();
}
</script>
</head>
<body>
<form name="form1" method="post" action="
[ Lien ]">
<input type="button" name="Submit1" value="Enregist1.asp" onclick="changeFormAction('Enregist1.asp?Id=<%=id%>');"><br>
<input type="button" name="Submit2" value="Enregist2.asp" onclick="changeFormAction('Enregist2.asp?Id=<%=id%>');"><br>
<input type="submit" name="bntAjouter" value="Ajouter">
</form>
</body>
</html>