So anyway from what I understood, it is a bad practice to try to divide the blockchain in 2 with symlinks or otherwise and I should keep everything inside a single HDD?
I've done this with many (non-Bitcoin) files for a long time: simlinking them to show up in several directories on different disks is very well possible. I used a script to create and delete all simlinks.But: it's very tricky, back in those days I was much more limited on disk space, and I wouldn't recommend doing it. In general, it's much, much easier to keep your data in one directory, unless it's absolutely necessary.
Linux doesn't care if blk00297.dat is a simlink to a USB-stick and blk00624.dat a simlink to a network mount, but if one of those is offline, Bitcoin Core does care.
