Attaching images to multiple galleries

If you open wp-includes/media.php file you will see such code in gallery_shortcode function:

...
if ( !empty($include) ) {
    $include = preg_replace( '/[^0-9,]+/', '', $include );
    $_attachments = get_posts( array('include' => $include, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );
    ...
}
...

It means that you can use gallery shortcode with include attribute, where you can insert ids of all images which you require for gallery: