I noticed what seems to be a flaw in my Kepler Bb humanoids' 2 circulatory systems. That would be bleeding. Sure, the 2 hearts get more oxygen so an MI is less likely and if it does happen, the other healthy heart compensates so they are only considered to have heart disease if both hearts are damaged in some way.
But there is a big problem here with having the arteries right next to each other. If you have a pair of arteries that are the same size and right next to each other, the force from blood pressure on the walls will change. The force from the right side of the left artery will cause the right side of the right artery to have more force. Likewise, the force from the left side of the right artery will cause the left side of the left artery to have more force. Maybe not double but definitely more. Also, the right side of the left artery and the left side of the right artery will have less force. So overall, the force is going to be going outwards. This will cause asymmetric vasodilation in almost all the arteries(I say almost because the aorta is elastic enough that this added force won't affect it much).
Vasodilation when not because of heat or infection is abnormal due to lowered blood pressure so the hearts will beat faster and harder to raise the blood pressure back up. This increase in heart rate and stroke volume is a problem because this will lead to more vasodilation and it is a vicious cycle. Eventually the arteries will tear. Once you get a tear in an artery, it can either continue down the artery until it reaches a branching point, tear back into the artery, or tear through all layers of the artery and cause internal bleeding.
This internal bleeding due to physiology is very bad news. It means that the humanoid will bleed to death no matter what is done to stop the bleeding initially. This is not a problem with the coronary arteries because 1 set goes from the top of the heart down and the other goes from the bottom of the heart up. This isn't a problem with the veins because of physiologically low blood pressure in the veins. And cappilary bleeding stops very fast and is nothing to worry about. So it is just the arterial bleeding that is worrisome.
So clearly something is needed to stop this vicious cycle. But moving the arteries so that they are further apart makes no sense(more space taken up by arteries than there needs to be) And having 1 heart only supply blood to 1 side of the body makes no sense either(this gets rid of the compensation benefit). So what else could stop this vicious cycle leading to bleeding?