(with-open-file (in1 "/etc/passwd")
  (with-open-file (in2 "/etc/group")
    (let ((in (make-concatenated-stream in1 in2)))
      (loop
        for pee = (peek-char nil in nil nil)
        for ree = (read-char     in nil nil)
        while (and (listen in) pee ree)
        collect pee into peeked
        collect ree into readed
        finally (assert (equal peeked readed))
                (return (list (length readed)
                              (+ (file-length in1)
                                 (file-length in2))))))))
--> (9543 9543)