That'd be better as

    (defvar foo-i 0)
    (defun foo ()
      (while-no-input
        (while (< foo-i 10000)
          (print (cl-incf foo-i))))
      (when (< foo-i 10000)
        (run-with-idle-timer 1 nil #'foo)))

Make a copy of all that, search-replacing "foo" with "bar". Run (progn (foo) (bar)). Both loops will complete eventually.

EDIT: OK, you explained what you call the normal way to do things. What kind of code is this that take >1 second in each repl? Out of curosity.