correct path for enqueue script in WordPress

Is your second WordPress installation accessible using URL? If yes then you can use the following tricks

add_action('wp_enqueue_scripts', 'op_enqueue_scripts');
function op_enqueue_scripts() {
    wp_enqueue_script(
        'my-script-from-2nd-site',
        'http://my-second-wp-installation.com/members/user.js',
        array( 'jquery' ),
        '1.0',
        true
    );
}