Hi Lionel,

The idea is to install an Odoo 9 community version first. After having this installed (for example under /odoo/odoo-server) you should create a separate directory for the Enterprise code. Make a new directory /odoo/enterprise/addons for example. You clone the enterprise code in to this directory and then finally you add multiple paths in the odoo-server.conf file. An example configuration file:

[options]
; This is the password that allows database operations:
admin_passwd = admin
db_host = False
db_port = False
db_user = odoo
db_password = False
addons_path = /usr/lib/python2.7/dist-packages/openerp/addons
logfile = /var/log/odoo/odoo-server.log
addons_path=/odoo/enterprise/addons,/odoo/odoo-server/addons
 #It is important that the path to the enterprise code is before the odoo 9 community path 
xmlrpc_port = 8069

Be sure that the path to the enterprise code comes before the path to the Odoo 9 community code.
If you want to you can use an automated script I've built that installs an Odoo 9 and then adds an Odoo 9 enterprise on top of the default Odoo. You can find my script here: https://github.com/Yenthe666/InstallScript/tree/Enterprise 

How to do this?
1) Create a new sh file:

sudo nano odoo_installer.sh

sudo chmod +x odoo_installer.sh

./odoo_installer.sh

The Odoo 9 community is under /odoo/odoo-server/addons by default. The V9 Enterprise code will be under /odoo/enterprise/addons/

Yenthe