Thank you. It works.
I now have a /etc/elasticsearch-node{n} and /var/log/elasticsearch-node{n} directories for each instances.
Had to chown elasticsearch. on these directories and create a service startup script for each instances in /usr/lib/systemd/system, and tweak the settings to point on the right dir.
Trivial actually. Sorry for not thinking about using different directory. It's the end of the day, getting tired I guess...