A pipe dream here would be to use both accelerometers at the same time and filter out much of the random noise resulting in much smoother accelerometer readings that could be used to calculate displacement.
(goes off and downloads Apple CoreMotion samples to see if they do this already)
If you mix a good quality accelerometer with a low quality one you get worse quality than the high quality alone.
The main problem that accelerometer have is that they are lineal sensors while human beings or animals use logarithmic ones.
What this means in practice is that it is very easy to saturate a sensor. You have sensors of 1, 2 or 3 maximun gs(gravity vectors of acceleration) while sudden movements could be bigger than that creating "out of range" samples.
What they probably do is use different scales for both accelerometers, giving them "HDR acceleration". E.g the low quality one at 10gs and the high quality at 2-3.
(goes off and downloads Apple CoreMotion samples to see if they do this already)