When you use two sabers, when calculating your damage, you take your strength modifier and add it to damage.
When you use either a single saber, or a double-bladed saber, you take your strength modifier, multiply it by 1.5, and then add it to damage.
For high strength scores, this is a significant improvement, way better than any crystal is going to help you.
*EDIT* - Anyone know what Final Lightsaber Form a Guardian/Assassin gets? Niman or Juyo?
*EDIT2* - Preliminary analysis shows that a double-bladed flurrying juyo form does the most damage...but a single-bladed POWER ATTACKING (of all things, I always thought this was the worst) juyo-form with a lightsaber built for criticals (Use the Improved Beam Gun lens) comes VERY close. Hmmm. Actually, I believe it depends on your strength score at the time...
*EDIT3* Well, according to my simple calculations (comparing two equal-levelled Jedi Guardian/Sith Maurauders), it turns out that a double-bladed lightsaber user using Juyo and Master Flurry will have a SLIGHTLY higher average damage per round than a SINGLE Lightsaber user using Master Power Attack and Juyo (Read - less than 10 damage per round average out of up to almost 500 damage per round). Single-Lightsaber style is VERY VIABLE for people who would prefer that (I am one of those people). Attack bonuses are also similar (Dueling vs the Advanced TWF chain). Power Attack has the cool knockdown effect, and more variable damage with its x3 crits, though. If you want to do MASSIVE damage in a single hit, you're best off with Power Attack, Shien-style and a keen Single-Bladed lightsaber (17~20/x4 damage...Terrifying).
Critical strike usually trails this by 40~50 damage a round, but the stun on Critical Strike is hard to ignore.