更新我已经对此进行了测试，并在我的网站上运行。有几件事：

更换我的$query 用你的global $wpdb （根据您关于全局变量的评论）因为它超出了范围

get_results() 如果没有其他说明，则返回一个对象（第二个参数是返回类型）

我将其放在插件中，但您可以提取代码并将其放在主题中，也可以将其放在functions.php.

功能如下：

function test_function() {

    global $wpdb;

    $query = "
        (SELECT * FROM wp_18_posts
        INNER JOIN wp_18_term_relationships ON wp_18_posts.ID=wp_18_term_relationships.object_id  
        WHERE post_type = \'post\' 
        AND post_status = \'publish\' 
        AND term_taxonomy_id = \'2\')

        UNION ALL

        (SELECT * FROM wp_17_posts
        INNER JOIN wp_17_term_relationships ON wp_17_posts.ID=wp_17_term_relationships.object_id  
        WHERE post_type = \'post\' 
        AND post_status = \'publish\' 
        AND term_taxonomy_id = \'2\')";

    $total_query = "SELECT COUNT(1) FROM (${query}) AS combined_table";
    $total = $wpdb->get_var( $total_query );
    $items_per_page = 1;
    $page = isset( $_GET[\'cpage\'] ) ? abs( (int) $_GET[\'cpage\'] ) : 1;
    $offset = ( $page * $items_per_page ) - $items_per_page;
    $latestposts = $wpdb->get_results( $query . " ORDER BY post_date LIMIT ${offset}, ${items_per_page}" );

    foreach ($latestposts as $latestpost) {
        $da_id = $latestpost->ID;
        $da_title = $latestpost->post_title;
        $da_content = strip_tags($latestpost->post_content);
        $da_content = wp_trim_words($da_content, 55);
        $da_link = $latestpost->guid;
        $da_date = $latestpost->post_date;
        $da_date = date(\'F j, Y\', strtotime($da_date));

        echo \'
        <div class="ldapost">
        <h2 class="lheader"><a href="\'.$da_link.\'">\'.$da_title.\'</a></h2>
        <span class="ldate">\'.$da_date.\'</span>
        <span class="lcontent">\'.$da_content.\'…</span><br>
        <a class="button btnright" href="\'.$da_link.\'">Continue Reading</a>
        </div>
        \';
    }

    echo paginate_links( array(
        \'base\' => add_query_arg( \'cpage\', \'%#%\' ),
        \'format\' => \'\',
        \'prev_text\' => __(\'&laquo;\'),
        \'next_text\' => __(\'&raquo;\'),
        \'total\' => ceil($total / $items_per_page),
        \'current\' => $page
    ));
}

原始帖子的paginate\\u links功能独立于您的查询。给定一些参数，如项目总数和当前页面，它可以提供您要查找的分页。因此，您需要计算：

项目总数当前页码，基于1的mysql limit语句的偏移量我在想这样的事情（未经测试，抱歉！）：

$query = "
    (SELECT * FROM net_5_posts
    INNER JOIN net_5_term_relationships ON net_5_posts.ID=net_5_term_relationships.object_id  
    WHERE post_type = \'post\' 
    AND post_status = \'publish\' 
    AND term_taxonomy_id = \'151\' 
    )

    UNION ALL

    (SELECT * FROM net_7_posts
    INNER JOIN net_7_term_relationships ON net_7_posts.ID=net_7_term_relationships.object_id  
    WHERE post_type = \'post\' 
    AND post_status = \'publish\' 
    AND term_taxonomy_id = \'20\' 
    )";

$total = $wpdb->get_var( "SELECT COUNT(1) FROM (${query}) AS combined_table" );
$items_per_page = 5;
$page = isset( $_GET[\'cpage\'] ) ? abs( (int) $_GET[\'cpage\'] ) : 1;
$offset = ( $page * $items_per_page ) - $items_per_page;
$latestposts = $wpdb->get_results( $query . " ORDER BY post_date LIMIT ${offset}, ${items_per_page}" );

foreach ($latestposts as $latestpost) {
    // Your code here ...
}

echo paginate_links( array(
    \'base\' => add_query_arg( \'cpage\', \'%#%\' ),
    \'format\' => \'\',
    \'prev_text\' => __(\'«\'),
    \'next_text\' => __(\'»\'),
    \'total\' => ceil($total / $items_per_page),
    \'current\' => $page
));

参考文献：

• This answer 关于分页
• This StackOverflow answer 关于MySQL联合中的计数