SBCL multiple threads write to standard-output

You need to pass the current *standard-output* to the new thread somehow, then in that thread's function, you can bind *standard-output* to that value.

Current Common Lisp implementations make thread-local dynamic bindings, and SBCL is one of them.

(sb-thread:make-thread ;; thread function
                       #'(lambda (standard-output)
                           ;; thread-local dynamic binding of special variable
                           (let ((*standard-output* standard-output))
                             ...))
                       ;; thread function argument, provided by the current thread
                       :arguments (list *standard-output*))

Note that I could have named the thread function's argument *standard-output* and then I wouldn't need the let, since the dynamic binding was done at function entry. But I think that dynamic bindings should be explicit and obvious, notwithstanding the earmuffs around special variable naming convention.