There are three kinds of weapons: main-handed, sub-handed and double-handed weapons. It is signified as M, S and D. Main handed weapons are equipped with the dominant hand, while sub-handed are on the non-dominant hand. Unless if the character has the "Double-Wielding" skill, it's not possible to equip two main-handed weapons.
However, equipping two sub-handed weapons are possible even without the skill.
Double-handed weapons use two hands, leaving the sub-hand unavailable to equip sub-handed weapons and shields. The only weapon with its official in-game descriptions are those usable in the dungeon. When chosen to unleash the hidden power, it will turn into Junk.