This game is glitched so that, if you use a double-sided lightsaber, you do not suffer the two-handed weapon penalty like you would with two separate lightsabers. Thus, to avoid the penalty, use a double-bladed one.
If you want to fight with just one single bladed saber, take the "Dueling" line of feats.
If you want to fight with a saber in each hand, take the "Two Weapon Fighting" line of feats.
If you want to use a double-bladed saber, don't take any line of feats, because 1) double bladed weapons are glitched in this game and your attacks are unpenalized when using them, and b) the "Dueling" feats don't apply to double bladed weapons.
I feel the end is near, my little Monday night whore. My little Saturday night, became a Sunday remorse.
Dueling does give you a defense bonus but you don't really need it at higher levels. I either use a double bladed saber to save feats or two set up for critical strikes. I usually take 2-weapon and flurry for a guardian but it also works with a sentinel.
The role of the weak is to serve the will of the strong; I'm glad you understand that. -- Canderous Ordo
While I personally love using two lightsabers I think we should be clear that the advantage of using two weapons or a doubleblade diminishes if you select feats or force powers that add attacks per turn (such as Master Flurry and Master Speed). All additional attacks per turn from feats and force powers are added to the main hand only. The off hand only attacks once, no matter what feats or force powers you use. Therefore, a character using two weapons (or a doubleblade) with Master Speed and Master Flurry will have 5 attacks per turn. Meanwhile, a character using one weapon with Master Speed and Master Flurry will have 4 attacks per turn.
All that being said, the off hand attack usually adds more damage than the +3 Damage bonus provided by Master Dueling. For example, with Master Speed and Master Flurry, the Master Dueling damage bonus adds up to +12 Damage per turn. So any off hand attack that averages better than 12 points of damage will generate more damage than Master Dueling. By the latter stages of the game an average of 12 points of damage is not terribly hard to manage with an off hand lightsaber (but that is likely at the cost of weakening Juhani or Bastila by hoarding all of the good lightsaber crystals).