its been said that he mastered both on several occasions and only he as far as i kno can call upon both sides simultaneously
OK now you are referencing the moment in the Revan novel. That moment has incredible similarities to Oneness with the Force. A brief moment where you obtain clarity within the Force. I have made several posts and a few threads as to how using both sides is impossible. I do not plan on reiterating myself.
Again you say 'its been said'. Unless it is stated in a canon source, it is not true. He was proficient with both sides of the Force. But he never used both of them at one time. Jedi Revan used Light Side. Darth Revan used Dark Side. Reborn Revan used Light Side, but fell back onto the Dark Side whenever he was stressed. He never mastered both sides. How many times must I say it? Luke and Sidious are the only people to ever master one side of the Force.