A Mam nivosa, I believe. I never saw a twoheader. Nice. Harry

That's what I thought initially too, and they look pretty close. However, my book says nivosa has 6-13 radials and yellow to brown spines, while this plant has white spines and 3-4 radials.

I had one similar that I identified as M. karwinskiana spp nejapensis. It was two-headed as well...

Jérôme

I think M. nejapensis is more likely. A relatively common plant that is renowned for producing one very long spine, lots of bristles, and sometimes dividing dichotomously.

Yep, I found some pics on mammillarias.net (pictures here) which confirm that it is M. karwinskiana ssp. nejapensis, aka M. nejapensis. Thanks folks!