From https://stackoverflow.com/questions/10376891/make-get-adjacent-post-work-across-custom-post-types

modified-get-adjacent-post.php

/*
 * Replacement for get_adjacent_post()
 *
 * This supports only the custom post types you identify and does not
 * look at categories anymore. This allows you to go from one custom post type
 * to another which was not possible with the default get_adjacent_post().
 * Orig: wp-includes/link-template.php 
 * 
 * @param string $direction: Can be either 'prev' or 'next'
 * @param multi $post_types: Can be a string or an array of strings
 */
function mod_get_adjacent_post($direction = 'prev', $post_types = 'post') {
    global $post, $wpdb;

    if(empty($post)) return NULL;
    if(!$post_types) return NULL;

    if(is_array($post_types)){
        $txt = '';
        for($i = 0; $i <= count($post_types) - 1; $i++){
            $txt .= "'".$post_types[$i]."'";
            if($i != count($post_types) - 1) $txt .= ', ';
        }
        $post_types = $txt;
    }

    $current_post_date = $post->post_date;

    $join = '';
    $in_same_cat = FALSE;
    $excluded_categories = '';
    $adjacent = $direction == 'prev' ? 'previous' : 'next';
    $op = $direction == 'prev' ? '<' : '>';
    $order = $direction == 'prev' ? 'DESC' : 'ASC';

    $join  = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_cat, $excluded_categories );
    $where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare("WHERE p.post_date $op %s AND p.post_type IN({$post_types}) AND p.post_status = 'publish'", $current_post_date), $in_same_cat, $excluded_categories );
    $sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1" );

    $query = "SELECT p.* FROM $wpdb->posts AS p $join $where $sort";
    $query_key = 'adjacent_post_' . md5($query);
    $result = wp_cache_get($query_key, 'counts');
    if ( false !== $result )
        return $result;

    $result = $wpdb->get_row("SELECT p.* FROM $wpdb->posts AS p $join $where $sort");
    if ( null === $result )
        $result = '';

    wp_cache_set($query_key, $result, 'counts');
    return $result;
}