My Kepler Bb humanoids have their arteries and veins doubled up(quadrupled if you are looking at the coronary arteries and veins). This makes sense because they have 2 hearts and both of those are 4 chambered hearts just like ours. If the arteries and veins weren't doubled up that would basically mean 1 8 chambered heart instead of 2 4 chambered hearts.
So since the 2 hearts are in sync with each other from embryonic development until a health problem affects 1 heart and the other heart compensates in response to it, it makes sense, to me at least that blood pressure would be increased even if left ventricular pressure stayed the same.
Assuming that the pressure-volume loop of the left ventricle and in fact all 4 chambers of the heart stayed as is, for the first few inches of aorta and the last few inches of venae cavae, the blood pressure would be the same. But most arteries and veins would not have the heart and their own muscular walls as the only sources of blood pressure.
You also have the fact that the arteries and veins are spaced so closely that they would push against each other as the hearts pump in or out of sync.
So the blood pressure in 1 artery would affect the blood pressure in the other artery. But lowering the blood pressure is just not feasible because the closer arterial pressure is to venous pressure, the less blood that flows and since less blood flow = less oxygen transport which is the exact opposite of why I doubled up most of the blood vessels in the first place(and quadrupled in the case of coronary arteries and veins), hypotension is just a no, regardless of how much it would lower pressure from arteries being next to each other.
So this leaves me with only 1 option I know of if I want to keep this double circulatory system. That is to raise the blood pressure in the arteries(so maybe what would be stage 1 hypertension for us would be normal blood pressure for my humanoids). This will increase blood flow because now there is more of a gradient between the arteries and veins. Bigger pressure gradient = more blood flow = more oxygen transport.
Since my humanoids naturally have a slower heart rate due to 2 hearts being in sync, this works well(at least I think it does). What is bradycardia for us is normal for them.
But would raising arterial blood pressure to increase blood flow despite the fact that now there is more force from each artery pushing on the artery next to it work to significantly increase oxygen transport(which is why I doubled things up(quadrupled for coronary))?