The bundling together of unrelated discoveries is weird, and I feel like it diminishes the impact of the prizes a bit in the public eye. (Instead of explaining one seminal discovery to the public, you have to explain two, and make it clear that they are not even related.) If anything, Nambu should have received his own prize, and then KM could have shared one with Cabibbo. But there are only a finite number of years, and particle physics only gets a prize at all every few years, so it is hard to reward all of the deserving subjects (if not people) without this type of bundling.