Apple is using two accelerometers in both the iPhone 6 and 6 Plus, Chipworks has discovered in its extensive teardowns of the two handsets. The first is the three-axis BMA280 accelerometer, made by Bosch, and the second is believed to be InvenSense's six-axis MPU-6700.
Why two? Interestingly enough, Chipworks believes that Apple decided to go with two accelerometers to improve power management and overall user experience. The InvenSense is more sensitive, and can do more things, than the Bosch, but it also draws a lot more power.