Since tag
is a taxonomy of a custom_post
post type, the query could look like this:
<?php
$args = array(
'post_type' => 'custom_post',
'tax_query' => array(
array(
'taxonomy' => 'post_tag',
'field' => 'slug',
'terms' => array(
'tag1',
'tag2',
),
),
),
);
$posts = new WP_Query( $args );
See WP_Query Taxonomy Parameters.