Oh, it seems to work like this:
In landscape orientation, only the ALS next to the camera is used, but in portrait orientation, both that ASL and the one on the side needs to be covered at the same time for screen dimming to happen.
Which is actually great news, since that means it doesn’t matter if you cover the landscape one while holding it in portrait orientation. 👏🏻
So much better solution than on my old 2018 Pro, where a single one was located pretty much in the top left (landscape) corner, where I often had my hand.