Andrew, if your Pie has an external controller you can simply disconnect the three thick phase wires
(Yellow, Green and Blue) to enable it to be pedalled normally. It sounds as if one or more of the
FETs have failed inside the controller.
Regarding the dual minis, if either the +5V supply to the throttle unit, or the hall sensor within the throttle, has failed, then both motors would stop working because there would be no throttle signal voltage coming from the throttle.
A faulty ground wire on the throttle would cause the motors to run at full speed if it occurs while power is already turned on, but it could also prevent the motors from working if the fault was there before the power was turned on.
I suggest you check the throttle connector to see if you have ~5v between the throttle red wire and the throttle black wire. If little or no reading is obtained, try putting the black probe from the meter to battery negative connection instead of the throttle black wire. If it then shows the correct ~5v, you probably have a break somewhere in the negative lead between the throttle unit and the controller.
Let's hope it's an easy fix once you've located the problem.
Alan
