Mongolian two tone singing is the practice of 'over-singing' in your chest voice so hard your vocal cords allow a 2nd or 'ghost voice' tone to be heard. It's a very difficult(and dangerous)technique. The 2nd tone gets louder as one learns to concentrate on how to let it through. Not at all similar to yodeling, which has two distinct voices. It's like humming and whistling at the same time. One air source-2 different places combine to get the sound. There was a time when I could do it, but my vocal coach assured me my career would be rather short lived, so I stopped the process. As far as anymore advice goes, I would recommend a complete loss of inhibitions, a life's partner with a high tolerance level and alot of practice and determination. A good friend told me when I started, "Yodeling is just one of things, as a singer you just 'have' to want to do well". Have fun, post back and let us know how it's going. You'll find other aspects of your singing will improve as you get to know the 'feel' of you own capacities.