Torque control of a BLDC motor acting as a generator
A dynamometer must provide negative torque, torque that opposes the motion of the shaft driven by the MUT. Dyno torque plus motor torque = zero.
The same parameter, torque or speed, can not be controlled by both the motor controller and the dyno controller. In this case, it is probably to control MUT speed and dyno torque.
The dyno controller is useless unless it can control negative torque and return energy to the source.
The easiest way to control the dyno is probably to not control the dyno motor at all. Just operate it as an uncontrolled generator. Rectify the generator output and use a chopper to control the load current. The current can be set manually or the current reference can be derived from a torque control error signal.