[Caden]

You still get 4 upgrades with a double-blade. The 2 crystals inside a double-blade work on both blades. So it's actually like having 4 crystals, even though you are only using 2 crystals. A rare, damage increasing crystal will effect both blades inside a double-blade. However, I don't think crystals that give bonuses to blaster bolt deflection will work on both blades inside a double-blade. Regardless, a double-blade is a good way of doubling your rare, powerful crystals.

Another difference is that double blades just do more blunt damage, while single-blades have a higher chance to land a critical hit. A double-blade with no crystals will do 2-20 damage and has a 5% chance to land a critical hit, with each blade. A single-blade with no crystals will do 2-16 damage and has a 10% chance to land a critical hit, with each blade. You mentioned that you have level 3 Critical Strike. That means you should be dual-wielding two single-blades. Master Critical Strike quadruples your chance to land a critical hit. This means a single-blade will have a 40% chance with each blade, while a double-blade will have a 20% chance with each blade. In your particular case, if you focus on increasing your lightsaber damage, you will definately deal more damage with two single-blades.