No, you’re right & I wasn’t thinking it through. At worst it will cause the core to fizzle due to radiation induced decays, but it shouldn’t detonate.
Still not great of course! Now you’ve spread the second bomb’s worth of Pu or U over a wide area with the energy of the first one. Toxic /and/ radioactive. What’s not to like!
(I may have been digging up a partial memory of muclear missile fratricide by Pu predetonation during compression due to the extra neutron flux, which IIRC is a real thing, but would require the second warhead to go through a full conventional explosion.)
