Client connection and server connection we need to connect with the same another conection server

We have a connection client (C) with a portal web (W1) through F5 and after this connection to the portal web the client have to authenticate inside of it in another server, authentication server (A1), after that client return to the portal web (W1) and there the connection works and it is working with persistence, but after that between two servers is necessary to validate the authentication, then the portal web (W1) have to check the authentication server (A1) but it will be balanced because there are more authentication servers and the balance could pass to another different and the connection to the authentication server will be lost, for it is necessary to keep the connection between client (C) and portal web (W1) and in the same time keep the same destination between the portal web (W1) and authentication server (A1), because is necessary keep that way.