Excellent question! I refer to them as a trimix diver or a deco diver which actually identifies what they do. "Technical diver" identifies them as doing "something, but we aren't sure exactly what" differently from most divers. What do they do differently? Are they wreck divers? cave divers? rebreather divers? trimix divers? The term isn't helpful in descibing what they do. If you want to describe their diving, use terms that actually mean something. The other problem (a minor problem, IMO, since the term isn't helpful anyway) is there is no standard definition. (As far as I'm aware, diving doubles isn't part of anyone's definition. From decades of personal experience diving doubles, I can testify it's not a big deal. In many ways, twins are easier to dive than a single tank.) Different agencies have different definitions. Dump the term.