Glycolic acid can only be combined with other exfoliating acids if a product contains a cocktail of different acids. This is because products that contain multiple acids are specifically formulated with low concentrations of the active ingredients so that they work together without causing irritation, On the other hand, products that contain a single exfoliating acid are usually formulated with a higher strength of the active, which shouldn't be layered with another exfoliating product in the same routine as it will most likely lead to irritation and can severely damage the skin.