An IMU is a single unit in the electronics module which collects angular velocity and linear acceleration data which is sent to the main processor. An IMU housing contains two separate sensors.
The first sensor is the accelerometer triad. It generates three analog signals describing the accelerations along each of its axes produced by, and acting on the vehicle. Due to thruster system and physical limitations, the most significant of these sensed accelerations is caused by gravity.
The second sensor is the angular rate sensor triad. It also outputs three analog signals. These signals describe the vehicle angular rate about each of the sensor axes. Even though the IMU is not located at the vehicle center of mass, the angular rate measurements are not effected by linear or angular accelerations.
The data from these sensors is collected by the IMU and returned to a main processor.
Because position is integral of velocity, and double integral of acceleration, minute offset voltages after calibration still accumulate relative position range or coordinate errors.
Without a GPS, or visual feedback, the flight time and integrated errors cannot be eliminated. Calibration is critical for longer flights and acceptable position error to get "home"
Thus averaging multiple IMU's only reduces the error by rms (n) or not much. (best case assuming random offsets)
"The Allan variance curves provides a tool for understanding the trade-off between repeatability (noise) and the averaging time.". Ref http://www.analog.com/media/en/analog-dialogue/volume-49/number-2/articles/mems-imu-gyroscope-alignment.pdf