There are several aspects which should be considered before deciding on whether layering is effective.
1. If melting garments (poly-cotton, polyester, acrylic, nylon, etc.) are used then the arrangement cannot be considered as arc rated. The use of melting garments are prohibited by OSHA and NFPA 70E.
2. Arc rated garments can be layered, but unless such arrangements are arc tested, a summation of the layers cannot be claimed. For example a 12 cal/cm² used over an 8 cal/cm² may not provide the same level of protection if the inner and outer layers are swapped around.
3. There are several factors such as breakopen and the let through energy (ATPV) that factor into layers from different suppliers.
4. Natural fibers can be used as inner wear or underwear, but NFPA 70E-2015 130.7 (C)(9) requires the main layer of protection to be adequately rated so as to not breakopen and expose the non-arc rated layers to the arc.
After all these factors have been carefully considered, layering (when done properly) can be very effective. When done in an uninformed manner, it can be more dangerous.