After many duels with lightsaber props, I can confidently say that the double-bladed lightsaber offers no tangible advantage over a single saber, and could be very disadvantageous against dual lightsabers.
It must always be held close to the body to prevent cutting one's self, only one end can be effectively utilized for attack and defense at any given moment, and to flow into using the second blade it frees up the opponent's own saber for use.
What you end up with is a single lightsaber that must be held close at all times.
Any benefits would come from an opponent who doesn't understand this.
It probably depends on multiple factors, a double bladed saber still gives you the ability to use the full strength of both hands in a swing, two sabers does not.
Two sabers gives you two lines of attack but also lessens your defense from styles like Djem So, so they all have their good points and bad points.