The only Surefire 2-stage tailcap of which I am aware is the McE2S tailcap. As far as I know, they're out of production now. Do a search on "McE2S" and you should find plenty of information.
These work on the Surefire E series body, and are intended for use only with LED heads. Incandescents don't like current reduced below specs, and won't function proprerly. But LEDs function just fine on reduced current; they just dim (and sometimes the tint shifts, but that's secondary). Lots of folks around here use the McE2S to turn their L4 and E1L lights into two-stage lights. I have both setups, and it increases the utility of the light significantly.
EDIT: Oh, and these are not manufactured by Surefire, but rather by a third-party. They are easily Surefire-level quality, but are not available through Surefire nor are they supported by Surefire.