1. Put pistol in pants
2. Put pistol in hands
3. Load pistol with ammo
4. Exchange pants you're currently wearing with another set of pants
5. While the pistol is still in your hands, try unloading the mag into your inventory
6. Take the pistol out of your hands
7. Take your pants back
8. Put pistol back in hands
9. Unload magazine
10. Voila, you have 2 magazines now.