Symfony2 - Multiple server application

I have an symfony-application(with login-system) running on one server.

Now i would like to expand my application on multiple servers. I read some article about doing this like:

Symfony 2 - multiple server configuration

Deploy Symfony applications across multiple servers

Would i learned so far:

1. First there is a webserver A, which only display´s basic frontend/login-form.

2. When user type´s credentials, the webserver A checks each other webserver if the entered credentials are correct.

3. If credentials are correct, the user gets a specific session to the webserver, where the user credentials are stored and user´s client can only communicate with this specific webserver.

4. If user log´s out session gets destroyed and user´s client can only communicate with webserver A.

Anybody has some suggestions and advices for dealig with this issue?

Thanks and Greetings!