...But what happens when your second magazine runs out? What then?
Let's say that I have a gun that normally has a seven round magazine. I add a second magazine. Now I've got 14 rounds! ...But why not just make a 14 round magazine? And now when I run out, I have to change two magazines. (FWIW, normal capacity for a semi-auto 9mm is 13-18 rounds, and 15 is about average. 7 is the OEM capacity of all traditional .45ACP 1911 pistols.)
IMO adding a secondary magazine, rather than increasing the capacity of an existing magazine, adds needless complexity. Added complexity increases the odds of failure. I hate to say that this is a skill issue, but, well, changing a magazine quickly is a skill that you should practice if you want to be proficient in practical shooting. (By "practical shooting", I mean things like IPSC, IDPA, USPSA, PCSL, three gun, steel challenge, self-defense, and war. You shouldn't need to worry about rapid magazine changes for hunting or long range competitions.)
Ennui and a gun.