How to combine two WP_Query objects?

I need to combine two different WordPress WP_Query Objects with separate $posts_per_page

lets say we have two different categories and each of them is responsible to show 18 posts or products of their own category, something like:

$category_query = new WP_Query([
   'posts_per_page' => '18',
    'post_type' => 'product',
    'tax_query' => [
       [
         'taxonomy' => 'product_cat',
         'terms' => 120,
         'field' => 'id',
         'include_children' => false
      ]
    ]
 ]);

Imagine Another query just like above but with a different term_id, as you can see posts_per_page is 18 and for each query 18 products with specific category will be returned.

Although it is correct, but I am trying to combine the two queries to reduce number of database queries and return the total of 36 products which the first 18 includes only category number 1 and 18 others for category number 2, even if they are in random order a simple traverse through array could fix this issue.

Do you have any idea how to do that?