Hey aprayoga, thanks for your answer, I would have answered earlier, but it seems that I can't post more than ONE (!) message per day on this forum, we need to find a better way to interact with users, because it is hard to both follow discussions and search in this one single immense thread.
BTW, following the fact that with a powered USB HUB I was able to see both disks, I took a cable with the double end (one for data and the other one for the power), connected both to the two back ports, and both disks are now usable. It is in any case strange, because I can connect these disks with just the single data cable to any raspberry and work on them, but for me this is an acceptable workaround, instead of playing with the /boot/armbianEnv.txt boot parameters.
RaSca got a reaction from Werner in Helios64 Support