An interesting idea, and one that has been implemented in other games. For example the Barb in D2, and in D&D characters are quite free to pack double weapons (any character).
I choose not to comment on how easy/hard this would be, as this is not for me to say. The only people that know that are ArenaNet. It could be as easy as tweaking a script, but it could need a complete game overhaul. I do doubt the former, however. It is possibly more likely in a future expansion, but I have my doubts as to whether we would ever see it.
Here is a novel idea I thought of though, if introducing slightly more complications for 'those who want to go double'. The first thing you have to follow is that sheilds are used as weapons and swords are used as protection. No debate, this is true; it's documented countless times in history. People didn't fix spikes to their shields for no reason, nor did they fit pomels to swords.
So, with that premise out of the way here is the idea. Everyone can take double whatevers. That means a character could take double swords; it also means they could take double shields. Double swords are obviously a lot more offensive, whereas double shields are very defense. But how the hell are you going to fight with double shields?? Please bear with me.
First off, other than 'handedness', normal 'sword-shield' (or whatever you use) fighting stays the same. However, in doubling combat it changes slightly. Using double swords you would not completely loose you a defense advantage, if nothing else you obsesive offense will protect you some, but you won't get full offense either (due to needing to protect yourself, offhand, etc.). So, based on the weapons stats, you will loose a nominal amount off offense and gain a nominal amount of defense, but still remain a highly offensive character. Similarly using two shields would result in a similar effect, though the other way round.
As I said, fighting normally would result in no change, so you wouldn't suddenly be able to fight with your shield as well as your sword. This would require more model animations however, as shield fighting doesn't work the same way as sword fighting from a human mechanics point of view. This does allow for far more flexability in styles of play though.
Similarly (as mentioned elsewhere) this could open the way to new skills, but there are a vast array of possibilities. For example you could gain more powerful spells by doubling, and these could also be dependant as to your style of play. Imagine, for example, a spell that allows a monk to deal a lot of health to the entire party, but requires him to be defensive. Or a violent firestorm brought on by an elementalist with dual wands.
There is an issue of balance amoungst classes. If a warrior suffered more damage through magical attack due to the reduction in AC it would work, but I don't think AC maters when magic is concerned (correct me if I am wrong).
Furthermore this idea is kind of complex, but the beauty of it is you can play normally without it being complex. If people want to take on the added complexity which fighting with more speciality brings (true also in the real world), then they deal with it. Furthermore it presents it for
all character builds, and allows more flexibiliy and further play options.
Just a thought. I don't know what you think, but I thought it seemed an interesting idea.
and then there's the months of testing yadda yadda....
