Merge Multiple WP_Query Based on Post ID

merge.php

<?php 

//setup your args with extra parameter fields => ids
$cat_query_args = array(
    'category__in' => 123,
    'fields' => 'ids',
);
$meta_query_args = array(
    'meta_key'		=> 'featured_post',
    'meta_value'	=> 'yes',
    'fields' => 'ids',
);
//setup your queries args
$cat_query = new WP_Query($cat_query_args);
$meta_query = new WP_Query($meta_query_args);

//now you got post IDs in $query->posts
$allTheIDs = array_merge($cat_query->posts,$meta_query->posts);

//new query, using post__in parameter
$query = new WP_Query(array('posts_per_page' => 4,'post__in' => $allTheIDs));

// from https://wordpress.stackexchange.com/questions/55519/can-i-merge-2-new-wp-queryvariable-s#answer-196161

?>