So it has to be two Force users? What, is an Old Republic council member not worth anything?
Don't discredit Sion... He survived the upheaval during the Sith Civil War on Korriban, and escaped onto Malachor V, where he was taught by a Sith more powerful and wise than Sidious.
Darth Maul was totally subservient to his master (Unlike a true Sith) and was murdered by a padawan.
Sion put Vash in the cage, using her as bait for the Exile. This shows that he succesfully was powerful enough to incapacitate(sp?) a Jedi Master and keep her subdued.
But more to the point, Sion would win. Maul is a mindless drone, or seems like one, and wouldn't be persuasive or smart enough to wear down Sion's hatred and will to live, which is the only way to kill him.