How do I display a tag cloud with both post tags AND a custom taxonomy?

Using wp_tag_cloud(), how can I display a single tag cloud that incorporates both regular post tags and a custom taxonomy?

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The following is a slightly modified version of the wp_tag_cloud() function:

function custom_wp_tag_cloud( $args = '' ) {
    $defaults = array(
        'smallest' => 8, 'largest' => 22, 'unit' => 'pt', 'number' => 45,
        'format' => 'flat', 'separator' => "\n", 'orderby' => 'name', 'order' => 'ASC',
        'exclude' => '', 'include' => '', 'link' => 'view', 'taxonomy' => 'post_tag', 'echo' => true
    );
    $args = wp_parse_args( $args, $defaults );

    $tags = get_terms( $args['taxonomy'], array_merge( $args, array( 'orderby' => 'count', 'order' => 'DESC' ) ) ); // Always query top tags

    if ( empty( $tags ) )
        return;

    foreach ( $tags as $key => $tag ) {
        if ( 'edit' == $args['link'] )
            $link = get_edit_tag_link( $tag->term_id, $tag->taxonomy );
        else
            $link = get_term_link( intval($tag->term_id), $tag->taxonomy );
        if ( is_wp_error( $link ) )
            return false;

        $tags[ $key ]->link = $link;
        $tags[ $key ]->id = $tag->term_id;
    }

    $return = wp_generate_tag_cloud( $tags, $args ); // Here's where those top tags get sorted according to $args

    $return = apply_filters( 'wp_tag_cloud', $return, $args );

    if ( 'array' == $args['format'] || empty($args['echo']) )
        return $return;

    echo $return;
}

Use the taxonomy argument:

$args = array(
   'taxonomy' => array( 'post_tag', 'custom_taxonomy' )
);
custom_wp_tag_cloud( $args );