Before you start, please make sure that you followed each step in the previous post.
Since we removed PGPORT variable in unit files (YAY!), the only thing that you should change is the port (unless you want to bind these instances to separate IP addresses...). Let's do it in a scripted way:
sed -i "s/#port = 5432/port = 5432/g" /var/lib/pgsql/9.3/data/postgresql.conf
sed -i "s/#port = 5432/port = 5433/g" /var/lib/pgsql/9.4/data/postgresql.conf
Great. Now 9.3 instances will use 5432, and 9.4 will use 5433. You can change them as you wish.
Finally, please start both instances:
systemctl start postgresql-9.3.service
systemctl start postgresql-9.4.service
You can verify this by connecting to instances using psql:
psql -p 5432 <-- will connect to 9.3 instance
psql -p 5433 <--- will connect to 9.4 instance.