The main heart is the command center for Red Blood Cells. The other heart is command center of White Blood Cells - the spleen.
The red blood cell heart is the nexus of the circulatory system that delivers nutrients and gasses.
But the white blood cell heart is the nexus of the another diffuse circulatory system - the lymphatic network. The White Blood Cell heart does not pump fluids. But you can think of it as the 'pentagon' of the human body has access to emergency to an reserve corps of Red Blood Cells if needed in an emergency like FEMA. Most importantly you can think of it as being high tech security center that trains White Blood Cells and Antibodies to attack intruders.
Regular Red Blood Cell Heart with four chambers is for the enlisted blood cells and the spleen is for the officers.