You loose one of your D from each attack.
6D in lightsaber, attack twice, each roll is made at 5D at difficulty 20.
I'm proposing you could use a double sabre, attack twice at 5D+5, DC 25.
OR
use a double saber, attack once and parry once and get a +5 on the parry.
The rules as written state you get a bonus to parry only, but that seems wrong to me somehow. The main advantage of the double sabre as I see it is how quickly it can attack, and that it can keep an opponent with a single weapon on the defensive. Maybe I need to adjust the bonus a little on the attack side?