I can do it pretty well -- the higher one, not the deep version (I can approximate the growly version, but it'll have me coughing after about 10 seconds, so I'm clearly doing something wrong!) -- after seeing Huun Huur Tu in concert back when I was in school I just tinkered until I could get the overtones reliably.

I also did a bit of barbershop (we called ourselves The Overhead Projectors...) and playing with throat-singing was actually quite useful in that context. I'm not sure how much it affect my actual voice control (probably some), but mostly I just became keenly aware of how much tone could vary just based on little changes in positioning of mouth, throat, tongue, palate, etc..