The trouble is the surge of current which fires the motion detector. In the old days, when computers were slow, there was enough time between the firing of the motion detector and when the force sensor was read that this was not a problem. Now computers are too fast.
Listed below is a workaround reported by one of our customers using a 500 MHz PC:
Place the Motion Sensor in port A, and the Force Probe in port C. Set the MPLI software up to think that there is a Voltage Probe in port B. This should allow you to collect both motion and force data without a problem.