Displaying a combination of Galleries

The built in shortcode doesn’t support that out of the box. However, it’s pretty easy to roll your own “multi-gallery” shortcode like so:

<?php
add_action( 'init', 'wpse36779_add_shortcode' );
/**
 * Adds the shortcode
 * 
 * @ uses add_shortcode
 */
function wpse36779_add_shortcode()
{
    add_shortcode( 'multigallery', 'wpse36779_shortcode_cb' );
}

/**
 * The shortcode callback function
 *
 * @ uses do_shortcode
 */
function wpse36779_shortcode_cb( $atts )
{
    $atts = shortcode_atts(
        array(
            'id' => false
        ),
        $atts 
    );

    if( ! $atts['id'] )
    {   
        // no list of ids? Just send back a gallery
        return do_shortcode( '' );
    }
    else
    {
        $ids = array_map( 'trim', explode( ',', $atts['id'] ) );
        $out="";
        foreach( $ids as $id )
        {
            if( $id )
                $out .= do_shortcode( sprintf( '
', absint( $id ) ) );
        }
    }
    return $out;
}

And of course, despite your best efforts to not use a plugin, here is that mess as a plugin. You could drop that code, minus the plugin header, in your theme’s functions.php file if you really really really don’t want to use a plugin.