How can I merge or combine facets?
I fact all you need is

hook_facet_items_alter(&amp;$build, &amp;$settings);

There you can change the queries and hide/show other facets like


      $build['A']['#markup'] = 'NewA';
      $build['A']['#count']  = $build['A']['#count'] + $build['B']['#count'];
      $size = count($build);
      $build['A']['#query']['f'][$size] = 'field_filter:B';