If all posts have the same template you could use this:
$posts = $site->index()->filterBy('template', 'YOURTEMPLATE')
Otherwise you could merge two collections like this:
<?php
$posts_a = $site->find('a')->children();
$posts_b = $site->find('b')->children();
$posts = new Pages(array($posts_a, $posts_b));
?>
Found here: Merge Collection of Pages