Combination of accelation signals from two devices
May be you can find variance of acceleration along x,y, and z direction in SET A and in set B separately and choose the the device which has least variance.



As this Wikipedia page suggests, you can combine the two by weighting by their respective variances.

So, to get $x_C$ simply:
$$
x_C = \frac{1}{\sigma_1^{-2} + \sigma_2^{-2}} \left(  \frac{x_A}{\sigma_1^2} + \frac{x_B}{\sigma_2^2}\right)
$$

Putting some numbers on this, suppose $\sigma^2_1 = 1$ and $\sigma^2_2 = 0.5$, then 
$$
\sigma_3^2 = \frac{1}{\sigma_1^{-2} + \sigma_2^{-2}} = \frac{1}{1 + 2} = \frac{1}{3}
$$
which is a lower variance than either of the individual measurements.

Use the same process to find $y_C$ and $z_C$.