Humanoid and Planet
I understand that you might think "2 circulatory systems require more oxygen so obviously it is not useful." but I know my Kepler Bb humanoids inside and out from anatomy and physiology(including how they form antivenom and antipoison via their own immune system) to development from pregnancy to adulthood.
I also know their planet inside and out(well maybe not the whole planet since I haven't mapped it all(haven't found a good 3D mapping software yet)) from their seasons and transition phases(like sprummer(transition phase between spring and summer)) and how long they last in kepler days and earth days(Kepler days are 1/3 longer than earth days) and thus the year length to the different biomes on this planet and what they are like during these seasons and transition phases.
Anyway here is the circulatory system in detail.
Humanoid Circulatory system
This actually comprises of 2 separate but connected circulatory systems. First off, their right circulatory system(which is only called that because of their right heart being the pump for it) is just like our single circulatory system with 4 pulmonary veins, a pulmonary artery, an aorta to the left, and venae cavae to the right and a 4 chambered heart.
Now their left circulatory system is inverted. The left aorta is to the right of the left heart. Right in the center of their chest are blood vessels connecting 1 aorta to the opposite heart via coronary arteries. So the left aorta supplies the right heart and the right aorta supplies the left heart. These are like backups in case the primary coronary arteries don't work but they also keep the heart strong, even in the worst cases of heart attacks and help the heart completely heal from a heart attack.
Now all of this means that the heart rate doesn't need to be as high to supply more oxygen as it does in the average human. The blood pressure is higher because of there being twice as many blood vessels(not counting the extra coronary arteries. Oxygen supply is higher and CO2 levels are lower. If 1 circulatory system shuts down completely, the humanoid will survive and still be healthy as long as their other heart is strong. So 1 circulatory system down does not mean heart disease.
Pros and Cons
Complete healing of heart No turbulence between circulatory systems Higher oxygen transport No heart disease unless both hearts are bad
Possible but unlikely, 2 different blood types in the same person. Makes blood transfusions extremely complicated since you would have to know which blood vessel is bleeding and thus which blood type to use on the spot and without testing, there is no guarantee. Heart problems more difficult to treat
So I see more pros than cons with this double circulatory system.
Kepler Bb is earthlike but with higher amounts of both oxygen and carbon dioxide(so a thicker atmosphere of basically the same air).
So is this double circulatory system actually useful?