How to add multiple checkbox elements to media attachments?

Like @Bainternet said, it is the same thing. Taking the code from the question you linked to, you can do it like this:

function filter_attachment_fields_to_edit( $form_fields, $post ) {
    $foo = (bool)get_post_meta( $post->ID, 'foo', true );
    $bar = (bool)get_post_meta( $post->ID, 'bar', true );

    $form_fields['foo'] = array(
    'label' => 'Is Foo',
    'input' => 'html',
    'html' => '<label for="attachments-' . $post->ID . '-foo"> '.
        '<input type="checkbox" id="attachments-' . $post->ID . '-foo" name="attachments[' . $post->ID . '][foo]" value="1"' . ( $foo ? ' checked="checked"' : '' ) . ' /> Yes</label>  ',
    'value' => $foo,
    'helps' => 'Check for yes'
    );

    $form_fields['bar'] = array(
    'label' => 'Is Bar',
    'input' => 'html',
    'html' => '<label for="attachments-' . $post->ID . '-bar"> '.
        '<input type="checkbox" id="attachments-'.$post->ID . '-bar" name="attachments[' . $post->ID . '][bar]" value="1"'.( $bar ? ' checked="checked"' : '' ).' /> Yes</label>  ',
    'value' => $bar,
    'helps' => 'Check for yes'
    );

    return $form_fields;
}