Im not an expert on acne and skin care but I think that its better to alternate between Benzyl peroxide and salicylic acid to keep the acne population down.

Generally, 1 in every 1,000,000 cells undergoes mutations that allow it to resist certain drugs. So if you use two drugs at the same time, it becomes 1 in every 1,000,000,000,000 cells that becomes resistant to both. I know that this is true for antibiotics but i dont know if it still stands for those medicated soaps. Maybe birdgirl can rain some knowledge on this.