I think for two nukes sitting any further apart than a few metres, if you set one off, the fissile material in the second nuke may have some small number of fis

I think for two nukes sitting any further apart than a few metres, if you set one off, the fissile material in the second nuke may have some small number of fissions due to incident neutrons, but it would be a very long way away from "most" of its energy. Just from a physics point of view, let's be generous and say that a single fission releases four neutrons, and I'm pretty sure that's an overestimate. If the fissile material in the second nuke has a radius of 0.1m (r), and sitting 5m (R) away, then it would be exposed to at a maximum πr^2 / (4πR^2) = 0.0001 of those neutrons. If we imagine that every single one of those neutrons causes a fission in the second nuke (and that won't be true either) then the yield from the second nuke will be less than a thousandth of the yield from the first nuke. Secondary fissions from the neutrons released from those fissions may increase this a little, but the second nuke is subcritical, so it won't be by much. I'd love to hear from someone who actually knows the answer for sure, though.