I use a double saber, simply because I tend to favor crystals which give nice damage bonuses ("your name" crystal + solari for instance), and since these crystals are unique, the most effective way to have the enormous attack and damage bonuses applied to both attacks is to use a double-bladed lightsaber, this also applies if I don't find enough good crystals to go around (the usual way things go).
On the other hand, for my jedi master builds I tend to favor stats boosting crystals such as Kaiburr and Ruusan which are not doubled in a two-handed lightsaber, therefore in order to maximize my bonuses, I'll opt for two single-bladed lightsabers (and probably go for defensive upgrades for the other parts of the blade). This way I can include as many wisdom, charisma, and defensive bonuses as possible in order to maximize my offensive power and staying power.
I've heard single +dueling feat allows a lot higher hit rate then double sabers, but since each blade has the same attack power, I find it hard to believe that a little extra hit rate will even out with an extra possible hit with every attack.
Outside of the game though, personally I think two double-bladed lightsabers are useable, though I consider the "effort in training"/usefulness factor dubious at best, except for use with the Tr