WordPress where clauses in db select

I think you are missing the relation key here. If you want the users to be picked up when both the above meta values are matched, then you should use this as below

$users_sent_to_admin = new WP_User_Query(
                        array(
                            'meta_query' => array(
                                'relation' => 'AND',
                                array(
                                    'key' => 'pending_approval',
                                    'value' => 'true',
                                    'compare' => '='),
                                array(
                                    'key' => 'is_photographer',
                                    'value' => 'true',
                                    'compare' => '='
                                ),
                            )
                        )
                    );