If your hair have already been relaxed, u absolutely cannot do this. The 2 chemical does not mix, your hair will break. I've relaxed my hair before (i'm not african by the way), but I hated how my hair is wavy so I relaxed it myself before, and after I got the Japanese straightening(thermal reconditiong), my hair is soo horrible that I wish I have my own natural hair back, it became kinky and break everywhere. later when I search more about it, it's said that you cannot do it if your hair has been relaxed because the chemical of the relaxer combine with the other one will cause damaged, unfortunately I didn't know about it until later. So I hope you don't do, the reason why alot of stylist that does this procedure does not recommend doing it on african hair is because almost all african women have used relaxer, so if a african women's hair has not been relaxed at all, then I'm pretty sure it can be done.
karen