This was the reply for whatever the worth
Obviously, when both engines are lost, and if enough fuel remains on board, the flight crew does everything to restart the engine using the windmilling. If it is not successful, the flight crew starts the APU and tries to restart the engines using the APU bleed. Therefore, if the engines are restarted, there is no need to use the Yellow Electrical Pump and the PTU.
If both engines cannot be restarted, we can identify 2 cases depending on the aircraft speed and the windmilling:
• We tried the Dual Engine Failure scenario in a simulator and the speed observed was between the ECAM recommended speed (280 KT) and the Green Dot Speed (220 KT in our example): The windmilling was enough to pressurize the green and yellow engine driven pumps. Therefore, in that case, thanks to the windmilling, there is no need to use the Yellow Electrical Pump.
• When there is not enough windmilling (it is the case in approach phase, for instance), the green and yellow engine driven pumps are no longer pressurized, and therefore the HYD G+Y SYS LO
PR alert will appear on the ECAM. This alert will request the flight crew to switch OFF the PTU and to switch ON the Yellow Electrical Pump (if yellow sys lost by ENG 2 PUMP LO
PR, which is the case in a Dual Engine Failure). This will enable to recover the systems supplied by the yellow hydraulic pressure, such as the alternate braking system.
For your information, if the Yellow Electrical Pump is started with PTU ON, then the PTU will run to pressurize the green hydraulic system. However, the flow delivered by the yellow electrical pump is low (32 l/min) compared to the flow delivered by the engine driven pump (140 l/min). Since the yellow pressure is used to pressurize the PTU, the yellow pressure drops down and by consequence the green pressure as well. Thus, if the PTU is not switched OFF, the HYD G+Y SYS LO
PR will remain.