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This seems a bug in the design of that REPL. Unless there's some reason the process has to be the same as the editor process.

I want multiple Lisp REPLs which run Lisp code concurrently in the same Lisp process.

How would I do it in GNU Emacs?

The first question is why. Can you not use CL repls instead of elisp repls?

I would want to hack Emacs Lisp, not Common Lisp.