You know, reading the little bastard's original post again on it, he might actually be talking about deep focus, but I still think he's referring to the thing I'm talking about. Could be either thing, since he did mention the "blur between the two figures" or whatever.
I'm talking about a case where a single frame seems to be divided in half, vertically, by an apparent change in focus. For example, in the left half of the frame, a character in the background is in focus, while his foreground is out of focus. Then in the right half of the frame (the same shot mind you, at least it appears as such if it is in fact an editing trick) a character in the foreground appears in sharp focus, while his background (thus the left character's space) in out of focus. It's not a case of deep focus where all layers of the frame are in focus, it's as though two halves of the same shot are both in shallow focus, merely focused in different planes.
It's a particularly jarring effect because where the two halves of the frame meet, there's a strange uncanny vertical line where the focus shifts from one side to the next. There's a particular shot like this in Serpico where I first noticed this being done. Like I said, I assumed this was what YLB was talking about, though I don't remember the shot in question in The Untouchables to confirm it or not, though no, it's certainly not the same thing as Welles' cabin shot.