Good question. Actually I have no idea about that but I will ask Tran Quang Hai if I'll see him the next time. Or maybe he will response here if he knows the answer. Have you seen Vladiwar Nadishana's well tempered Kou Xiang with seven blades. But what 's the principle ? sure that it should sounds good :)
I meant - how the fundamental notes of each blade are selected to provide an overall overtone harmony? Also, somewhere I read that the Chinese (or occidental?) music scale is different from European one, so which one is used there?