You should put a pistol magazine on the opposite side of your body from the gun, whether pocket or belt carried. The gun takes up enough space in the pocket without adding a magazine to it. (Which is why I dislike pocket holsters that incorporate a mag pouch.) As well, when you need to reload, you want to keep the gun in your shooting hand while pulling the magazine with your support hand. This will be awkward or next to impossible if the mag is on the same side.