Linus claims that even if stacking radiators is not the most optimal configuration, it is a valid way to install them when no other space for a fresh-air radiator exists.

“optimal would be every radiator having access to fresh air that's optimal what they said is that it will not help or it could even make it worse so what we tested was if it helps at all and of course it does now there are a variety of reasons why it helped for us and did not help in corsair's flow s”