The potential transformer and the current transformer provide differential outputs that have galvanic isolation from the voltage and current in the circuit they are measuring. However, there is no need to run the outputs of these transformers into differential inputs of the data logger and unnecessarily consume additional data logger channels. We conducted extensive testing for noise immunity, for inaccuracies from ground loops, and more before concluding that single-ended measurements in the ACPower() instruction have the same performance as differential measurements would provide. Please note that as a result of the galvanic isolation of the potential transformer and current transformer, the data logger ground is NOT connected to the ground of the circuit they are measuring.
Said differently, you can connect differential outputs of a sensor to single-ended inputs of the data logger. However, doing so creates the possibility of poor common-mode noise rejection in the data logger and the possibility of introducing inaccuracies from ground loops between the sensor and the data logger. Note that in this application, the transformer isolation of the potential transformer and the current transformer eliminates these concerns.
Simply connect one of the potential transformer secondary wires and one of the current transformer secondary wires to the data logger ground. Which wire in either case makes a difference, as the phase information allows the measurement of power flowing in either direction. If you measure negative real power when it should be positive, then reverse the secondary wires of the potential transformer where they connect to the data logger. Alternatively, you can reverse the secondary wires on the current transformer, but don't reverse both pairs of wires.