The only advantage it has against multiple opponents is that there are 2 blades to block the attacks of both opponents simultaneously.
But it's movements are restricted with one blade always being opposite the other. I would think Ventress's Jar Kai is more useful against multiple opponents, given how she consistently fights off both Kenobi and Skywalker simultaneously, each one of them being at least on par with her in a one on one.
Whilst all Maul did with his Saber Staff was fight off 2 opponents each one of whom was below him in combat prowess anyway.
I'd say the main advantage of a Saber Staff is it's an unorthodox fighting style which the opponent may find confusing at first. Plus the fact that you can always switch back to a Single Saber which could really confuse your opponent. But that will only work if you've mastered both styles. Opress for instance only seemed comfortable using a Staff and not so much with a Sword.