I currently carry the extra mag for my LCP in my left front pocket in a small baggie with a rubberband to hold the bag on. Haven't found a mag pouch I like yet.
Unless I'm missing your setup, that seems a bit complicated. Under zero stress, perfect weather conditions and in the light, I imagine you can get the rubber band off and the extra mag out of the baggie as long as the threat waits for you to do so or is otherwise engaged and not paying attention to you after unloading your first magazine at em.
Seriously, that's not good advise to give someone asking what to do with their extra magazine. Reminds me of the Archer episode where Archer used an empty Baby Ruth candy bar wrapper as a condom. There are much better and proper options to recommend.