Fast destructive list splitting in Common Lisp

split-list.lisp

(let* ((list (list 1 2 3 4 5 6 7 8 9 0))
       (first list)
       (second (cdr list)))
  (do ((left list (cdr left))
       (right (cdr list) (cdr right)))
      ((and (null (cdr left))
            (null (cdr right))))
    (setf (cdr left) (cddr left)
          (cdr right) (cddr right)))
  (values first second))

;; => (1 3 5 7 9)
;; => (2 4 6 8 0)