1) Have no magazine/drum loaded into your gun
2) Put your only magazine/drum into a protector case
3) Scroll wheel to reload the magazine into your gun
4) You'll then have one magazine/drum in your gun and one in your protector case. The game will then think it's only one magazine/drum (For example if you load the magazine/drum in your gun and shoot both magazines will have 1 less bullet)
5) When you have one magazine/drum in your protector case and one in your inventory you'll then have to re-log
6) Once re-logged you will have TWO magazines/drums.
Video steps: https://www.youtube.com/watch?v=xO_dlkSyhrc