How to talk in two different voices at the same time without electronic help?

I was wondering how i'd be able to them talk in two different pitches at the same time. The reason i'm interested is because i'm currently starting an un-black metal band (have just about everyone i need) I play guitar and I'm pretty good at low growls and medium range to somewhat high vocal distortion. I've been told it sounds similar to Dani filth's vocals (from Craddle of Filth) even though I don't try to imitate him I don't even like C.O.F that much. I want to learn how to growl or atleast talk in two different voices (kinda like what most would consider to sound like a demon). I'd really appreciate any real help I can get so no sarcasm please i'd rather for you not to answer than to say something just to try to annoy me. I know it can be done because I was friends with a guy that could do it awhile back. Thx in advance