Finally it's not so necessary for my company, my colleagues prefer to separate customer demands in another project.
Then is simplifies the view for developpers and managers.
So that's good I can go on with chiliproject :)

For people who are interested in :

We can create sub projects.

For example when you create project : "Project A" and user 1, 2 and 3 in this project.
Then you create sub project "Project A_in" and user 4 and 5 in it.
All demands declared by user 4 and 5 are visible by user 1, 2, 3, 4, and 5.
But demands declared by user 1, 2, or 3 in the project A are visible only by users 1, 2 and 3. But users 1, 2, and 3 can also create demands in project "Project A_in" visible by all the users.

"Project A_in" is used by customer, and "Project A" is used by the company who develops the software.
I've just found this possibility, it seems to answer to my initial post.

I hope this will help others.

For the record: it is currently not possible to have "instance-based" rights that differ from the rights you have for other instances of the same object in a project. In other words: it is not possible to have extra/other rights on an issue than you have for all other issues in a given project.

The accepted solution to your problem is indeed having 2 projects: one public and one private.