Re: placement of our body parts
heart comes with extensive plumbing. I case of second heart, you would need bigger body to fit all those veins.
In case of injury person would more likely to die, because now they have more places whey they can bleed to death.
Heart has some fluid dynamics too, he is the only boss pushing blood around, in case of 2 hearts still one heart would decide the flow.
Or they would start pushing each other.