Putting a fan after another will up pressure (doubling in theory), but not displace more air. Except if the air has to go through something slowing it down where more pressure will help to push more air through it.

There are obviously lots of variables at play (the extra fan takes up space too, reducing airflow for example) so you are surely right that it's probably useless or worse to have a "double fan" for a PC cooling system, but the fans being in perfect sync is just a noise problem (on small fans like this).