[Al Skierkiewicz]

We calibrate speed controllers each year in the current robot, to be sure they are working properly with the interface and all parts. I have never heard of a team trying to calibrate two speed controllers at the same time. At best this sounds like a dangerous practice. It is best to remove all circuit breakers except for the the controller that is being calibrated. Then you must remember that the controller does not store the calibration points until the button is released. The cal button must be pressed and held while the joystick is moved from extent to extent and while it is allowed to rest in the center position. We will not release the cal button until a "hands off" (no one touching the stick)verbal acknowledgment has been received.
Can I assume that since these controllers are feeding the same gearbox that you are running them from the same PWM output? Is your mechanical design capable of running with one motor disabled? Of course, if one motor is calibrating and the other motor is powered down, there should be no problems. I suspect that what may have happened is that one of the cal buttons was momentarily released during the cal process and it would have tried to supply full output to a locked gearbox.