I was thinking the same, given that Teller, Sakharov et. al. came up with, but discarded as unworkable, several designs (the "Classical Super", the "Layer Cake"...) before they discovered that, unfortunately, there was a way to make it work. My guess is that the X-ray pulse from a buried bomb would be quickly absorbed (it is quite rapidly attenuated just by air), and that whatever X-rays reached the second bomb would all be from one direction, ruining the symmetry that is apparently needed for fusion ignition.

On the other hand, I have heard that a non-trivial part of the yield of a hydrogen bomb comes from the fast neutrons from the fusion causing a much more complete fissioning of the fissile material. Maybe, if the buried bomb was not damaged to the point where it was incapable of fusion ignition, the second bomb would contribute to the explosion in this way, without acting as a hydrogen bomb itself. With a high enough neutron flux from the first bomb, maybe the core of the second one would not have to undergo implosion, or even stay intact.

The article also talks about a 17-mile kill zone, and the creation of a new "North Carolina bay" despite the crash being 50 miles from Pamlico Sound. These seem to me to be incompatible claims, and surely the second, at least, must be hyperbole?

https://www.britannica.com/technology/nuclear-weapon/The-fir...

Richard Rhodes, "Dark Sun."