You have your normal application, accessible on some port serving HTTP requests (or whatever). Then you start a thread (with `bt`, bordeaux-threads, a very common multi-threading library) and run a Swank server listening on some other port on it. Swank is the backend used by SLIME, it allows manipulation of the Lisp environment. A remote server is still the same Lisp environment as when you run it on your machine, so you can use Emacs to connect to it (M-x slime-connect) and debug live.

Swank doesn't allow remote connections though, otherwise anyone could connect and mess with your server. You must connect through the loopback interface. Easiest way is to set up an ssh tunnel, like: ssh abc@example.com -L 4200:localhost:4200

Whether you can call the Swank server a second server or not is a somewhat philosophical question I guess. There's one application serving different kinds of requests on different ports.