test.el

(defun print-plist (a)
  (loop for (key value) on a by 'cddr
        do (message "key: %s - value: %s" key value)))

(defun merge-plist (first second)
  (loop for (key value) on second by 'cddr
        do (setq first (plist-put first key value))))


(setq a (list :foo 1 :bar 2))
(setq b (list :foo 2))
(message "Before:")
(print-plist a)
(merge-plist 'a b)
(message "After:")
(print-plist a)