(defun deepth (list)
  (if (atom list)
      0
      (1+ (reduce (function max) list :key (function deepth) :initial-value 0))))

(defun flatten-before-deepest (list)
  (case (deepth list)
    (0 list)
    (1 list)
    (2 (flatten list))
    (otherwise (mapcar (function flatten-before-deepest) list))))

(flatten-before-deepest '(a (b (c) d) e ((f g) h i) j))
;; --> (a (b c d) e (f g h i) j)