You can connect to multiple nREPL servers using M-x cider-jack-in (or cider-connect) multiple times. To close the current nREPL connection, use C-c C-q (cider-quit). You can close all connections with C-u C-c C-q.

CIDER maintains a list of nREPL connections and a single 'default' connection. When you execute CIDER commands in a Clojure editing buffer such as to compile a namespace, these commands are executed against a specific connection.

You can display the current nREPL connection using C-c M-d and rotate the default connection using C-c M-r.

Switch to connection buffer

The REPL buffers double as connection buffers.

To switch to the relevant REPL buffer based on the Clojure namespace in the current Clojure buffer, use: C-c C-z. You can then use the same key combination to switch back to the Clojure buffer you came from.

The single prefix C-u C-c C-z, will switch you to the relevant REPL buffer and set the namespace in that buffer based on namespace in the current Clojure buffer.