Not only should the frequencies be harmonic, but they, Idealy, should be the same, to get the best preformance. I've heard of harmonic coils set up, but for unipue purposes. For instance, i read of one where two coils were set up one was set a frequency of 2/3 the other, and every 6 cycles (or somthing like that) there was a really big spark between the two toploads.