It doesn't make any sense, all Zoro's attacks he targets the upper torso.
But it depends on the situation, sanzen sekai also targets the chest, but Zoro used it to hit Oars in the belly, used the shishisonson to attack the PH Dragon on the neck and etc.
Against Kaido it's the same thing, when he and Killer combined their attacks, directly showed the mark on Kaido's neck, it wouldn't make sense to show only the mark of Killer's damage, and hide the mark of Zoro's damage.
In addition, when 2 characters come together to damage an opponent, they always come together to apply damage in the same area.