Probé esto en FF 3.0.11 e IE8, y me funcionó:
Código HTML:
<script>
function actionForm(formid, act)
{
document.getElementById(formid).action=act;
document.getElementById(formid).submit();
}
</script>
<form id="form1" method="post">
<input type="button" value="Send!" onClick="actionForm(this.form.id, 'pag.php'); return false;" />
<input type="button" value="Send2!" onClick="actionForm(this.form.id, 'pag2.php'); return false;" />
</form>
Nos vemos.