There's a big difference between swapping out an existing weapon with pre-existing variables attatched, and creating a new weapon every time you feel like turning off a blade. A switch weapon button would be considerably easier, because you already have the weapon. It's in your inventory, it's got the variables already attatched to it.
To make a light saber dual bladed and make it so you could turn off 1 blade, either they'd need to have a hidden weapon attatched to your pc and swap between the 2 (and that could get REALLY buggy, like if you have more than 1 2 bladed lightsaber, and you turn them both to 1 handed, the variables would stack up quickly and lag your machine in the way described below, plus the act of making the weapon visible and the old weapon invisible, not to mention glitches that would happen if there was too much in your inventory. Would you lose the lightsaber? If you've got too many processes going on at once would you get a clone in your inventory? can you drop it or equip it, or is it just there taking up space? this list could continue. ) or you would create the new weapon every time you "turn off the one side" and just have to carry the variables over to the new weapon every time, but that as well is open to glitching, on a slightly lower scale, and especially during combat the act of applying the variable to the pc, then creating the weapon, then applying the variable back to the weapon, would create a hell of a slowdown.
Either way from a programming standpoint it would be hard. I'm sure they could figure out a way to do it, but it's going to be really complex for a small feature not a lot of people would use. after all, they give out lightsabers like candy in KOTOR 1.
Why not just make it so you can swap out gems on the fly, using a repair skill? It would be easier to program and accomplish the same effect.