From Sven Hodapp on the user mailing list:
I've read in the Accumulo Book (p. 496) that it should be possible to start on a (fat) machine multiple tablet servers to scale (also) vertically. Sadly it's not described how to do it. Also I didn't find anything about this issue in the official documentation.
It would be good for us to document this and keep it up to date (as it does change from 1.6 to 1.7 to 1.8).