Hi @herrfischer
Hmm… I believe you should be able to accomplish this using a tax_query in WP_Query. For example, the following code will query posts that are in the red and vegetable category:
$args = array(
'post_type' => 'post',
'tax_query' => array(
array(
'taxonomy' => 'category',
'field' => 'slug',
'terms' => array('red', 'vegetables')
),
),
);
$query = new WP_Query( $args );
For more information, have a look at: https://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters
Hope this helps 🙂