[myform]
    Foo: [mytextbox name="foo"]
[/myform]

function shortcode_myform($atts, $content = null) {
   return '<form action="#" method="post">' . do_shortcode($content) . '</form>';
}
add_shortcode('myform','shortcode_myform');

function shortcode_mytextbox($atts) {
   return '<input type="text" name="'$atts['name']'" />';
}
add_shortcode('mytextbox','shortcode_mytextbox');

function attribute_map($str, $att = null) {
    $res = array();
    $reg = get_shortcode_regex();
    preg_match_all('~'.$reg.'~',$str, $matches);
    foreach($matches[2] as $key => $name) {
        $parsed = shortcode_parse_atts($matches[3][$key]);
        $parsed = is_array($parsed) ? $parsed : array();

        if(array_key_exists($name, $res)) {
            $arr = array();
            if(is_array($res[$name])) {
                $arr = $res[$name];
            } else {
                $arr[] = $res[$name];
            }

            $arr[] = array_key_exists($att, $parsed) ? $parsed[$att] : $parsed;
            $res[$name] = $arr;

        } else {
            $res[$name] = array_key_exists($att, $parsed) ? $parsed[$att] : $parsed;
        }
    }

    return $res;
}

[outer_shortcode]
    [inner_code url="#" title="Hello"]
[/outer_shortcode]

add_shortcode('outer_shortcode',function($atts,$content) {
    return attribute_map($content);
});

add_shortcode('inner_shortcode',function($atts,$content) {
    return '';
});

Array
(
    [inner_shortcode] => Array
        (
            [url] => #
            [title] => Hello
        )

)