Glamdring, the sword that Gandalf uses, is a single blade and is a different kind of weight system entirely. You're trying to compare lightsabers to swords, which just isn't possible. The hilt of a lightsaber has got to be quite heavy considering it has to generate that kind of energy to make the beam. And then we're talking about a wooden staff and a metal sword. Different technology.
This is something I grabbed off of Wiki:
"A lightsaber is a difficult weapon to master, mainly because there is very little weight or substance to the blade, making it almost seem unbalanced. Attempting to use it as one would use a sword will result in uncontrollable swings that are just as likely to damage the wielder as the opponent. Using one effectively requires precise measurement of force with no over-swing."
I would bet money that you would more likely chop your arm off if you tried to balance a double-bladed in one hand and then go ahead and swing with a single blade. It's just so different with the weight issue. On one hand you've got a heavier weapon (the double) and in the other a lighter one. It'd be confusing and more dangerous to the wielder. IMHO, it's not worth it and wouldn't work out. Sorry.