App note from Kionix on utilizing 2 linear accelerometers to determine angular rotational rates. Link here (PDF)
In many applications, customers would like to measure rotational motions (angular velocity, angular acceleration) in addition to linear motions. Most often, gyroscopes are added to their end product to obtain the rotational information. In some instances, customers already have a system that contains two or more accelerometers. With some understanding of fundamental physics, they can extract more than just linear acceleration data from their system.
from Dangerous Prototypes http://bit.ly/2QGsgYv