Show posts by category combination

‘category__and’
https://codex.www.ads-software.com/Function_Reference/WP_Query#Category_Parameters

$wp_query->query(array('category__and' => array(7,9),'paged' => $paged));

Perfect! Thank you.

One more question for you…

I would like to limit the number of posts this query displays, which is different from the rest of the site (8). I would like to limit it to two posts shown.

I tried this with the help of the link you provided but it didn’t work

<?php
$temp = $wp_query;
$wp_query= null;
$wp_query = new WP_Query( 'posts_per_page=3' );
$wp_query->query(array('category__and' => array(7,18),'paged' => $paged));
?>

try:

<?php
$temp = $wp_query;
$wp_query= null;
$wp_query->query(array('category__and' => array(7,18), 'posts_per_page' => 2, 'paged' => $paged));
?>

Yes. Again, Thank you!