You have a choice - either keep the number 1 engine running in accessory drive to pretty much guarantee being able to get home or shut down completely and hope the battery has enough oomph to get the first engine started.
All it needs is wet or failed start and suddenly the volts are dropping quickly - we start the number one in manual control as a precaution against low voltage causing fuel computer problems.
Sometimes the choice is dictated by how long the winchman will be handing over the casualty to the hospital staff - we try to get our kit back, especially stretchers, and it may take a while for the casualty to be in a stable enough condition to be safely removed from our stretcher.
Bizarrely, we only have one battery fitted to the Mk3 and 3A when there is appropriate provision for a second one.
We do know it pisses people off listening to jet noise in the middle of the night but a. we have to make sure we can get home and stay on operational state and b. jet noise is the sound of freedom so tough!