The simplest answer to that is, "Jedi". The longer answer is that the more skilled one gets, not only with using the lightsaber, but also in connecting with the crystal inside of it, the flow of energy gets easier to manage and "lighter" for lack of a better word.
Yeah, I've heard that. But what I was actually wondering was whether wielding two swords at once (particularly two heavy swords) was something that would be practical in real life, as opposed to just being a fantasy action-movie trope that looks cool but would really be clumsy and useless.