override Wordpress core public function - wordpress

Question

I have 2 separate wp installs that are sharing the same database. My goal is to use the same post table and post meta table for both websites.
I have achieved that by adding in my config.php of the second site, the code:
define( 'CUSTOM_POST_TABLE', 'site1_posts' );
define( 'CUSTOM_POST_META_TABLE', 'site1_postmeta' );
Also I have added in wp-includes/wp-db.php , public function tables, line 1026-1076 the code
if ( isset( $tables['posts'] ) && defined( 'CUSTOM_POST_TABLE' ) )
$tables['posts'] = CUSTOM_POST_TABLE;
if ( isset( $tables['postmeta'] ) && defined( 'CUSTOM_POST_META_TABLE' ) )
$tables['postmeta'] = CUSTOM_POST_META_TABLE;
Now both websites share the same posts but the issue is that the next time wordpress will be updated, I will loose the changes from wp-includes/wp-db.php
So I tried to override the public function with another function inside the function.php file of second website Override arguments of public function in Wordpress but the website crashed and returns 500 error. Propably I am doing sth wrong.
The code I tried is
add_filter( 'tables', 'my_tables', 11, 2 );
public function my_tables( $scope = 'all', $prefix = true, $blog_id = 0 ) {
if ( isset( $tables['posts'] ) && defined( 'CUSTOM_POST_TABLE' ) )
$tables['posts'] = CUSTOM_POST_TABLE;
if ( isset( $tables['postmeta'] ) && defined( 'CUSTOM_POST_META_TABLE' ) )
$tables['postmeta'] = CUSTOM_POST_META_TABLE;
}
return $tables;
}
The original code inside wp-includes/wp-db.php is
public function tables( $scope = 'all', $prefix = true, $blog_id = 0 ) {
switch ( $scope ) {
case 'all' :
$tables = array_merge( $this->global_tables, $this->tables );
if ( is_multisite() )
$tables = array_merge( $tables, $this->ms_global_tables );
break;
case 'blog' :
$tables = $this->tables;
break;
case 'global' :
$tables = $this->global_tables;
if ( is_multisite() )
$tables = array_merge( $tables, $this->ms_global_tables );
break;
case 'ms_global' :
$tables = $this->ms_global_tables;
break;
case 'old' :
$tables = $this->old_tables;
break;
default :
return array();
}
if ( $prefix ) {
if ( ! $blog_id )
$blog_id = $this->blogid;
$blog_prefix = $this->get_blog_prefix( $blog_id );
$base_prefix = $this->base_prefix;
$global_tables = array_merge( $this->global_tables, $this->ms_global_tables );
foreach ( $tables as $k => $table ) {
if ( in_array( $table, $global_tables ) )
$tables[ $table ] = $base_prefix . $table;
else
$tables[ $table ] = $blog_prefix . $table;
unset( $tables[ $k ] );
}
if ( isset( $tables['users'] ) && defined( 'CUSTOM_USER_TABLE' ) )
$tables['users'] = CUSTOM_USER_TABLE;
if ( isset( $tables['usermeta'] ) && defined( 'CUSTOM_USER_META_TABLE' ) )
$tables['usermeta'] = CUSTOM_USER_META_TABLE;
}
return $tables;
}
How could override it so I won't loose my changes after the next update?