Sorry if this has been covered already, but I didn't come up with anything after a quick search in the forums and browsing tag information.

Is there an established convention for tagging two-speed fans? I'm wondering how to treat the cmd and speed aspects of these.

I think the fan cases covered right now are simple on/off or on a VFD. I don't know that I have the right answer for you, but you could consider using speed with discrete percentage values, or could consider using run + stage (1,2) along the lines of cooling/heating equipment.

If its just one point, then I'd probably use an enum point:

point
kind:"Str"
enum:"off,slow,fast"