you can dum noobs you have to do a quest first -.-
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If you have a weapon in your inventory, click on it to wield it. This will un-wield any other weapon you have in your weapon slot.
It saves a spot when smithing because you can wield it.
In the current game you cannot dual wield. However it was confirmed in one of the behind the scenes video's that dual wielding would be introduced as part of the evolution of combat update.
In order to wield dragon weapons, you need to have high enough levels (usually level 60) and in most cases a quest. To wield different weapons, you need to do different quests. For instance: to wield dragon long-swords or daggers, you need to do the lost city quest. but in order to wield the dragon scimitar, you need to do the monkey madness quest. The runewiki has a list of the quests required to wield the different dragon weapons.
Heroes quest, which also supplies you with the ability to weild a dragon battle axe.