Im not exactly a programmer, much less a game designer, but if this were to be implemented, I'd see one of two possible ways.
1. The thorough way - you would have to create the dual wielding mechanic, so that two weapons of an allowed weapon type could be equiped simultaneously. Personally I'd create a new weapon type - daggers - just for this purpose. Daggers would be much weaker than 1 handed swords, so that adding their attack values wouldn't become a problem (read overpowered). I'd also adjust the blocking mechanic for dual wielding: normal blocking wouldn't work at all, only perfect blocking would be allowed.
2. The slightly but not much easier way - just create a new "dual daggers" weapon type. Gameplay wise they are a single item with less attack power but higher attack speed than 1 handed swords. Since they are a single item, a shield could be used just like with any of the current weapons.
No matter which way you like more, there is no way around creating new sprites for dual wielding. I'd like to add though that, while dual wielding certainly is a cool idea, I don't care that much about it right now. I just want the game to be finished. Once that is done, I may start dreaming about cool new fighting styles. ^^