Single Java Backend Multiple Frontends

I have a scenario like I have multiple frontends for each user which will connect to a single backend(initiator) through web socket, and that backend will communicate with a system(acceptor) through a socket connection. As the user is more than one we can't create multiple instance of the backend. Any idea you can suggest other than multi-threaded programming?

Note: Each time an user will come and specify the connection details of the system at 3rd level(acceptor), and backend(initiator) will create a connection with the system.