Having several layers of chemical peels is not an additive effect. Many times your doctor or esthetician will make several passes during a chemical peel and each pass does tend to penetrate deeper because the earlier passes broke the bonds between the cells. This allows deeper penetration. However, they are not additive. To get a 30% TCA peel you need to use 30% TCA and at this level should be performed by a dermatologist who is skilled and trained in medium depth peels to avoid risks of scarring or other complications.

Shawn Allen, MD Boulder Dermatologist

http://www.realself.com/question/layers-15-tca-peel-in-sitting-equal-30-tca-peel

That's all I could find from a professional. I thought it was like a percent increase. I guess this makes sense, it's always the same percentage but when you impact the skin with the first layer the second is more harsh on the broken skin. That's how it feels when I do two layers.