Sorry I am thinking doppleganger which has the two dots over the a. Bloody Germans.
Without being too technical, of course the two tones produce a doppler effect. There is of course "two tones" (i.e two seperate pitches, or two seperate wavelengths) in two tones, hence the name. I believe I am right in explaining this for long straight ranges.
Kempie, with respect, you are confusing the issue, but likewise due to my simplification, I know where you are coming from.