Large double bits were developed for men who built skid roads in the logging industry. By nature this work involves a lot of grubbing out of low lying wood, roots, deadfall, etc. And at the same time large live trees needed to be cleared. A full sized double bit can do this. That type of work is rarely done by hatchet wielders.
Not to say that a 2nd sharp bit on a hatchet wouldn't be handy at times. It would be - especially if you don't get around to sharpening your axe very often. But I would personally find more use in a poll or spike.