No you dont have to be ambidextrous..... most of the time dual wepons are sharp on the one side, blunt on the other, that way you can use it both as slashing weapons and as shields.....
And since i can fight with 2 katanas (no i am not extremely good with it, but i can kick a monster's but at least.... dont give me 2, but you get MASTERS at it)
The basic trick when fighting with dual weapons, is that one weapon always acts as a shield, while the other attacks..... the nice thing is it takes you a slit second to "change" them... and even then, dont think your "left" weapon will act the same as the right.... your main hand, slashes, the other hand mostly pokes... its STILL very effective.... but then again, i have seem masters at "play" where you cant see which hand is the main.... so with practice, you DO become ambidextrous with the weapons - not nescesarily with a pen aswell