Submit posts in two different post types with the same form

Your best bet would be hooking into the post type transition and creating the second (custom) post (type) via PHP.
Here is a function that can accomplish just that:

// Automatically create a news (regular post in category ID 1 = news) when a wiki post (custom post type) has been published
function nxt_create_news($new_status, $old_status, $nxt_wiki_post) {
    if('publish' === $new_status && 'publish' !== $old_status && $nxt_wiki_post->post_type === 'wiki') {
        $nxt_post_author = $nxt_wiki_post->post_author;
        $nxt_wiki_permalink = get_post_permalink($nxt_wiki_id);
        $nxt_wiki_title = $nxt_wiki_post->post_title;
        $nxt_postarr = array(
            'post_author' => $nxt_post_author,
            'post_content' => 'The wiki entry ' . $nxt_wiki_title . ' has just been published.<br /><br /><a href="' . $nxt_wiki_permalink . '" title="' . $nxt_wiki_title . '">Check out wiki entry</a>.',
            'post_title' => 'The wiki entry ' . $nxt_wiki_title . ' has been published',
            'post_status' => 'publish',
            'post_category' => array(1),
        );
        $nxt_post_id = wp_insert_post($nxt_postarr);
        set_post_thumbnail(get_post($nxt_post_id), '1518');
    }
}
add_action( 'transition_post_status', 'nxt_create_news', 10, 3 );

For you, it might be the other way round. You’d create a custom post type after a regular post in some specific category has been created. I hope the example above provides enough context, but just ask if you need more input.

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