Wouldn't need to as the RAT would deploy for the loss of either. I have on a few occasions, observed hydraulic pressure indication from a windmilling RB211
Engineers are warned that hydraulic pressure will move control surfaces even when dry spinning an engine using the starter motor.
A 747-400 pilot has even reported getting full electrics from a windmilling engine (but that was at cruise speeds). The crew of the BA 747 which flew through volcanic ash were also getting intermittent electrics (not just Battery/Standby power). You wouldn't be getting electrics from unpowered engines windmilling at 140kts, however (assuming the 777 was approaching at around this speed)....and you would be probably getting close to losing hydraulics for flight control. However, as repeated ad nauseum... no one is saying that the engines were windmilling.