While most of us possess two hands, some more than that, we are usually more skilled at using one than the other. In many cases, the offhand is only a spectator while the other hand performs most duties. The same applies to wielding weapons, our primary hand is much more adept when using weapons. Through rigorous training, one can become skilled enough to use weapons in their offhand(s). If you are really good, you can probably hide that you have an offhand at all.