Already from a probability point of view one can argue that the standard deviation of the mean drift goes down when stacking multiple IMU's together. However, things might also turn out nastier when the odds are not in your favour (e.g. in the unlucky event that both IMU's have exactly the same drift).
There are ways in which you can benefit much more from having multiple IMU's though, rather than just stacking them together. If placing them on different locations of the body, then you end up with more measurements than system states (i.e. an overdefined observation space). These different measurements can be combined using observers (e.g. by using a Kalman filter) to get much more accurate results.
Consider a 2D pendulum in a gravityless environment of length $r$ with two IMU's, located at $r/2$ and at $r$ from the joint to which the pendulum is connected. The centripetal acceleration measured at $r$ should be double that of the one measured at $r/2$ (i.e. $r\omega^2$ instead of $r\omega^2/2$, where $\omega$ is the rotational velocity of the pendulum). If it's not, then you know something is wrong. A bit of smart observing will (in most cases) help you track down the error.