buffer-flippy.el

(defun mjk/buffer-flippy ()
  (interactive)
  (let ((now (cadr (current-time)))
        (then (or (bound-and-true-p mjk//last-flippy-time) 0))
        (prev-counter (or (bound-and-true-p mjk//flippy-counter) 0)))
    (setq mjk//last-flippy-time now
          mjk//flippy-counter (+ 1 prev-counter))
    (if (> (- now then) 2)
        (setq mjk//flippy-counter 1))
    (if (<= mjk//flippy-counter 2)
        ; replace with `other-buffer' if you don't want window-local history?
        (spacemacs/alternate-buffer)
      (previous-buffer))))

(defun spacemacs/alternate-buffer (&optional window)
  "Switch back and forth between current and last buffer in the
current window."
  (interactive)
  (let ((current-buffer (window-buffer window)))
    ;; if no window is found in the windows history, `switch-to-buffer' will
    ;; default to calling `other-buffer'.
    (switch-to-buffer
     (cl-find-if (lambda (buffer)
                   (not (eq buffer current-buffer)))
                 (mapcar #'car (window-prev-buffers window))))))