🇷🇺Russia @pvasili

…
  $query1->union($query2, 'UNION ALL');
  $query1 = $query1->extend('PagerDefault');                  
  $results = $query1->execute();
…

we can try to do

…
  $query1->union($query2, 'UNION ALL');
  $query_pagination = db_select($query1,'q')->fields('q');
  $query_pagination = $query_pagination->extend('PagerDefault');
  $results = $query_pagination->execute();      
…