I've been using Epiduo for two months now, and I'm just seeing the end infamous "purge" (I hope!). I've added a vitamin C serum as I've read online that it's great with Benzoyl Peroxide, but I'm wondering if there is anything I can do to help the process.
My skin is combination-oily and I've been breaking out on my chin and cheeks for a couple of years now.
I wouldn't add vitamin C at the moment - it can add to skin irritation, and you skin is trying to find its peace with the retinoid in the epiduo.
Instead, I'd consider adding an azelaic acid 10-20%, for example https://whatsinmyjar.com/product/dermatica-azelaic-acid-20 (they ship to different countries I believe) or https://whatsinmyjar.com/product/the-ordinary-azelaic-acid-suspension-10
If you can, try to cleanse in the PM in one step instead of double cleansing (or only double-cleanse in the areas where you use stubborn makeup like mascara).