I agree with the first answer, but to expand : the left ventricle pumps oxygen rich blood to the body (through aorta) and the right ventricle pumps the deoxygenated blood to the lungs.
Anyways the reason two hearts aren’t necessary is because you have more than one chamber in the heart. The left ventricle is bigger with thicker walls because it pumps blood throughout your entire body. The right is smaller because it just pumps to the lungs and back.
