Sometimes if a person has sensitive skin, then consult a dermatologist before experimenting with active ingredients. But if you have a good skin tolerance, you can alternate two different activities like a morning product and a night product. For example, you can go for the BHA on the first day and the AHA on the second day, and so on. Always remember not to combine two or more acids within the same session, resulting in severe irritation if you have sensitive skin. Similarly, certain combinations will render one or more ingredients non-effective. For example, when you combine benzoyl peroxide with tretinoin, the letter will be rendered ineffective due to the properties of benzoyl peroxide. Unless the doctor prescribes a combination, always try to avoid them.