The PTU should not be running during 2 engine ops on taxi out if the G and Y Hyd sys are operating normally.
it is triggered to work automatically when the sys pressure difference between these 2 are greater than 500 PSI.
Without being a mechanic I would say something was not operating normally if both engines were indeed running and if the PTU was still barking.
Single engine taxi and forgetting to turn on the Y electric hyd. pump would trigger it to work until you set the parking brake.