Take the same two drivers loaded in either separate horns or one horn with a double the mouth area of the first example and you will get 6dB additional- 3dB from the extra power handing, 3dB from the increased radiation efficiency gained by doubling the radiating(mouth) area.

In short the Wicked One isn't quite what the author of the piece thinks it is. Is it better than the same horn with one driver? Yes. Equal to the same two drivers in two horns of the same size? No.

As to the JBL example, the shortening of the horn loses 3dB, which by doubling the drivers is gained back. The bad news is that the shortening occurs at the throat end of the horn, not the mouth, so the actual overall cabinet volume reduction is not 33%, it's more like 10% at best, and you've incurred the cost of a second driver. If you really need the space and are willing to spend the extra money then fine, but that pretty much goes counter to the usual intent with a horn.

Bottom line: there still isn't any free lunch.