Attaching projects to one window causes the index to be shared.

I have multiple projects that live in one codebase, this is for a frontend / backend php application. The project folder contains the sub projects, and some docker-compose files to start up everything.

project
* docs
* frontend
* backend

Previously, I had two projects, one for frontend and one for backend, since if I just created a project for `project`, the code in frontend would auto-complete with classes from the backend and visa versa.

Since PHPStorm 2013.3.2, it is possible to attach projects so they are in the same window. This seems a great idea for this setup, but it seems that the autocompletition for project A is again showing classes of project B, even though they are separate projects.

I also tried to setup a new project (on the directory of `project`), and assigning `frontend` and `backend` projects to that. They would display in the code tree at the left, but I still have the same issue that both projects share a index so autocompletition of the backend shows frontend classes.

The documentation states that "Symbols from the added projects are visible from the primary one, but not vice versa." (https://www.jetbrains.com/help/phpstorm/2016.3/opening-multiple-projects.html#d942944e18), but this doesn't seem to be the case. 

Is this correct behavior?

Regards,

Michaël