如何使2个网站使用相同的帖子表

I have 2 different websites installed on the same database. The 2 websites are sharing the users tables, but I want them to share the posts tables too. I tried to do it by editing wp-db.php, using a method I found from some old posts. After this code:

foreach ( $this->tables( 'global' ) as $table => $prefixed_table )
$this->$table = $prefixed_table;

I added this code:

$this->posts = 'wp_' . 'posts';
$this->postmeta = 'wp' . 'postmeta';

But it doesn't seems to work.

I need to make both websites to use the same posts and postmeta tables instead of every site using it's own posts and postmeta tables. How can I make the 2 websites use the same posts and postmeta tables?

Thank you!