If you are going to do this, I strongly suggest that you keep your Barney mag in a different pocket from your starting mag. Otherwise, you are likely to mix up the two and start a stage with your Barney mag (which may be a round or two down from division capacity).
I found it easier to use a third mag carrier, just to avoid the extra digging around in my pockets and to reduce the chance for confusion. Also, I might have stuffed a partly empty magazine in my pocket from a previous stage, and I don't want to accidentally pull that out and use it during load and make ready. That's why I use a third mag carrier.
Another tip: never put a partly loaded magazine into a mag carrier. At the end of the stage, when you unload and show clear, take that mag out of the gun and stuff it in your pocket. Similarly, when you are picking up your empty mags from the stage, stuff them in your pocket. Don't put them in your mag carriers. Otherwise there will come a time when you start a stage thinking that the mags in your carriers are filled to division capacity but they are partly or completely empty.
If a magazine is in my mag carrier, it is at division capacity and ready to go.