Right now the way to do that is creating multiple Odoo instances with different config files that hold different addons_path options. I know that Odoo will release in a few months a tool or something to be able to manage different addons_path using only one instance or at least that's what I understand from a chat with Fabien. If not I will build my solution for that but right now you need to create several instances to have different addons_path. A few good practices for that:

- Keep the code for each database only for that database, if you use git you could clone the git repo for every database to be sure that an update in one database doesn't corrupt the others

- Use the dbfilter option to match, show and manage only the database of the instance or create a PostgreSQL user for each Odoo instance if you don't wanna see all the other database in the Odoo instance

- If you are using Odoo 8 or 9 you need to configure the longpooling port different for every database.