Problema con form e doppio submit

[fermat]

ecco qua (gli ho dato una pulita dalle cose inutili):

<form method="POST" action="<?php $_SERVER['PHP_SELF']; ?>">
    <table>
    <?php
    if ($_SERVER['REQUEST_METHOD'] == "POST" && isset($_POST['btn_r_sconto'])) {
        // FAI UNA COSA
    }
    if ($_SERVER['REQUEST_METHOD'] == "POST" && isset($_POST['btn_qta_articolo'])) {
        // FANNE UN'ALTRA
    }
    ?>     
        <tbody>
            <?php
            foreach (................ as $art): ?>
                <tr>
                    <td>
                        <div class="has-success">
                            <input type="hidden" name="r_id[]" value="<?php echo $art['r_id']; ?>">
                            <input type="text" name="r_sconto[]" value="<?php echo $art['r_sconto']; ?>"> - 
                            <input type="submit" name="btn_r_sconto" value="Modifica sconto">
                        </div>
                    </td>
                    <td>
                       ..........
                    </td>
                    <td>
                        <div>
                            <input type="hidden" name="articolo_id_qta[]" value="<?php echo $art['fk_articolo']; ?>">
                            <input type="text" name="qta[]" value="<?php echo $qta; ?>"> - 
                            <input type="submit" name="btn_qta_articolo" value="Modifica qta">
                        </div>
                    </td>
                    <td>...........</td>
                </tr>
            <?php endforeach; ?>
        </tbody>
    </table>
</form>