Is there a way to order posts and custom post types as one group?

In terms of the query, it’s pretty straightforward to do this, but you’ll have to come up with your own interface for creating the order of posts, which should ultimately save the post IDs to an array in the order you want them to appear:

$post_ids = array( 42, 13, 23, 99 );

Then it’s just a matter of getting a “page” of those IDs:

// current page number
$paged = 1;
// number of posts per page
$posts_per_page = 2;
// starting position
$offset = ( $paged - 1 ) * $posts_per_page;
// extract page of IDs
$ids_to_query = array_slice( $post_ids, $offset, $posts_per_page );

Then pass those IDs to the query via post__in, and orderby the post__in array:

$args = array(
    'post_type' => array( 'post','secondposttype' ),
    'post__in' => $ids_to_query
    'orderby' => 'post__in',
    'ignore_sticky_posts' => 1,
    'posts_per_page' => $posts_per_page
);
$ordered_posts = new WP_Query( $args );