UNION queries don't support PagerDefault

…
  $query1->union($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();      
…

Try this

$query1 = db_select('a_blog', 'cb')
->fields('cb', array('title', 'changed_date'));

$query2 = db_select('a_gallery', 'cg')
->fields('cg', array('title', 'changed_date'));

// Ensure both queries have the same number of columns and name them consistently.
$query1->addField('cb', 'title', 'title');
$query1->addField('cb', 'changed_date', 'changed_date');

$query2->addField('cg', 'title', 'title');
$query2->addField('cg', 'changed_date', 'changed_date');

// Combine the queries using UNION ALL.
$query = $query1->union($query2, 'UNION ALL');

// Extend the combined query with PagerDefault.
$query = $query->extend('PagerDefault');

// Execute the modified query.
$results = $query->execute();