I would like to know if I could still write in
/home/usr and have the storage used from the new one
No, because the disk is currently mounted at /hdd, not at /home/usr.
If you want to use the HDD for /home/usr, you should first move all data from the existing /home/usr to the HDD, then re-mount the HDD there.
(That is, if the disk is suitable for use with /home – i.e. fast enough that it won't get annoying. Personally, I would continue to use an SSD for /home and keep the HDD at /hdd, creating symlinks like "/home/user/Movies → /hdd/Movies" for convenience.)
mount the new one at / and use it as it is. I found that there should be no issue to have two HDD mounted in the same dir from from this post, is it true?
Yes and no. It's true in the sense that it won't damage anything… but that doesn't mean it'll do anything useful at all. So mostly "no".
In Linux (as in most other systems), mounts are opaque. If you mount disk 2 at
/, then only the contents of disk 2 will be visible at
/ – the previous contents of disk 1 will be covered up and inaccessible. (In fact, the
/bin/umount command will become inaccessible as well, so the only thing you can do is reboot.)
(Hence the suggestion to move data instead of copying.)
However, it may be useful to do the opposite: you can mount the same HDD on two directories. (This is usually done indirectly via "bind mounts" which start from an existing mount, but I'm quite sure Linux allows starting two mounts from /dev/sdX as well.) For example, if you want to keep the new disk to be mounted at
/home/usr and at
/hdd at the same time (e.g. for organizational purposes), that's possible.
Finally, if you want a "merged" mount that allows accessing files from both disks at the same location, that's technically possible – but I wouldn't recommend doing that for /home. You could, however, use something like MergerFS to combine e.g. /hdd1 and /hdd2.