Oscar, With your bass tuned to pitch: capo the 1st fret on the B string while holding down the 15th fret at the same time. While holding down at both of these frets: Slide a thin guitar pick between the top of the 8th fret and the bottom of the string... see if the pick slides in cleanly and just fitting. This is a great starting point for neck relief that is recommended by a few makers. Since you have some back-bow on the B-side you could loosen that rod a 1/4-1/2 turn.
Hi KR
Thank you for the advice. As per a luthier and repairman who looked it over, I used the wrong word, so I apologize! If I were to make an adjustment, I could introduce some back-bow, but the luthier and repairman advised me to leave it as is, because the current difference in relief on the bass side is only slight, and, as per him, it is accommodating for the added movement of the B-string, and if I, or he, were to make it 1:1, I would instead have the entire string B string buzz against the frets. I think the problem here is that:
A) it's the first time I have an instrument this expensive in my hands, so I'm quite paranoid about any type of deviance
B) it's the first time I play a bass that has dual truss rods, so I have all sorts of worries in my head (if there is anything in there?), a lot of them probably unfounded, but better safe than sorry!
C) I had this worry that any alternating action would warp the neck long-term, and not considering that (and I hope I am correct here) the two truss rods are there for extra stability, to minimize the chance of warpage, and to allow greater flexibility in adjusting the setup to what suits me. In short: There is no right or wrong, as long as the differences aren't very big between both sides.
D) it's a dream for many people to own an Alembic, so I also want to honor that by taking great care of the instrument

Sorry for the wall of text to everyone so far! I hope it's okay - else let me know, and I will try to shorten my replies.
Kind regards
Oscar