WP_Query get post from a category and from another post type

A good way could be the one explained here

Basically you get the different queries results separately, assigning them to two different variables, merge the two result sets into array, getting the IDs of all this combined arrays, and last.. querying by IDs

$args1 = array(
  'post_type' =>   'help',
  'post_status' => 'publish'
  )
$args2 = array(
  'post_type' =>   'post',
  'cat' => $categoryId,
  'post_status' => 'publish'
)
$help_posts = get_posts( $args1 );
$post_posts = get_posts( $args2 );
$combined_query_array = array_merge ($help_posts, $post_posts ); //merge
$post_myneeded_IDs =  wp_list_pluck( $combined_query_array, 'ID' ) //squeeze IDs out
$desidered_query = get_posts( array(
  'post__in'    => $post_myneeded_IDs,
  'orderby'     => etc. etc
));

update, following the comments.. using ‘fields’ => ‘ids’

$args1 = array(
  'post_type' =>   'help',
  'post_status' => 'publish',
  'fields' => 'ids'
  )  //now get_posts() will return just a collection of IDs
$args2 = array(
  'post_type' =>   'post',
  'cat' => $categoryId,
  'post_status' => 'publish',
  'fields' => 'ids'
  )
$help_posts_ids = get_posts( $args1 );
$post_posts_ids = get_posts( $args2 );
$combined_posts_ids = array_merge ($help_posts_ids, $post_posts_ids ); //merge
$desidered_query = get_posts( array(
  'post__in'    => $combined_posts_ids,
  'orderby'     => etc. etc
));