[evo]

Right! But that's talking about compressing a plutonium pit in atmospheric pressure--being an extremely dense metal, plutonium will preferentially "squirt out" in any direction rather than compress, if there's a direction it can go in. Therefore, you need to make a spherical shockwave via explosives.

But that's in 1 atm! The initial wavefront of a fission device is going to be conservatively about 4 inches a microsecond (based on early above-ground test photos, modern high-yield devices would probably be faster still). This could very well turn the entire physics package of the secondary weapon into a thin pancake on the blast front, the plutonium can't get out of the way fast enough to avoid compression. This is a totally different scenario than a one-point-safety fizzle. The HE explosives in the second bomb might as well not exist relative to the overpressures faced from the first bomb's detonation.