If you can choose between one massive impact and two half-sized impacts, surely the two half-sized is better? The air can absorb more of the impact on each smaller rock, and the dust kicked into the atmosphere would be less than half for each smaller impact (right? wrong? correct me, please).

Hard to say how many rocks you could break a big asteroid into, but I suspect it would still be a much more optimal outcome.

Plus, we could convince the major superpowers to fire all their nuclear arsenal into space. I am all for that.