Because the essence of twin sword fighting is not to hit your weapon with your other weapon, which is remarkably easy to do- in fact, it can be hellishly difficult to cover your middle with two weapons when the other one gets in the way.
Two weapon fighting is often with two short blades, as they are less likely to get tangled, but elsewise you would use one long blade and one short one.
Fighting with two blades, regardless of the size (and you guys will just have to trust me on this one), would be very hard as you for one thing wouldn't have as much power behind attacks as you'd only being using one hand for each weapon, while with one weapon you could hold it with both hands.
To be honest, out of the lightsaber styles (1 blade, 2 blades, or the lightstaff) I think the lightstaff would, while being a little tricky to get used to, would probably be the most effective.
And before anyone asks how I know these things, I was on colorguard. Our swing flags were about the right length to be a lightsaber and our regular flags were just about the right length to be a lightstaff. We'd duel with them, and since I'm...well, me, I tried out fighting with 1 swing flag, 2 swing flags, 1 regular flag, and 1 regular and 1 swing flag. It was quite fun, actually. Lots of random injuries, though.
Err... yes...
But actually the Lightstaff is a totally useless weapon, SS! Do remember it is not a normal hafted weapon- it's too dangerous. It would be insanely dangerous to use and would almost certainly lop one of your own limbs off before it hurt an opponent, and if an opponent struck at your guard you are totally buggered. Sabre combat is vicious that way; small touches make big differences. Tap, say, a quarterstaff with a sword and nothing happens; touch a lightstaff with a lightsabre and it breaks. Likewise, whilst you might brusie yourself with a quarterstaff, you will lose your head with a lightstaff- and nor can you move your hands along its length, another important feature of hafted weapons.