Single sabers have been shown to use multiple crystals in the EU.
Corran Horn makes a saber that has 2 crystals in it and by twisting the hilt he changes what crystal is used to make the beam which changes its colour and length. Another student pulls it off as well with three crystals which allow him to have three lengths and a multi-colour rainbow effect.
Thus it has been established that single handed sabers alone can use multiple crystals and it can cause them to increase length, width and colour.
Would be logical it can be done with dual blades as well which could be rather interesting from a visual perspective but ingame its not going to have much impact on gameplay.
Unless your gonna do something really stupid like a toggle ability which suddenly increases all saber attacks to 10 meters because the blade is super big...
Would be more reasonable to attach a grenade launcher to the lightsaber or... turn it into a Andromeda Swiss-army Force Lance