In theory, the smaller shot provides greater pattern density while the larger shot increases downrange lethality -- it's win-win. But in practice, some ammo makers (and this amateur handloader) have encountered difficulties. Consider a two-layer duplex load. If the large shot exits first (as is most typical), you can end up with severe pattern stringing: The thinly spread larger shot reaches the target first, followed shortly by the rapidly decelerating smaller shot. Reverse the shot layers, and the big shot overtakes the small shot midair. Pellets collide. The pattern disintegrates. Cats living with dogs. It's ballistic chaos.