All jet2 737 a/c have a gpu connected before engine shut down, even if the apu is working. It's another way to save on fuel and on apu wear.
Normally one engine is shut down when on stopped on stand, then the second once the ground power is attched. Onboard the a/c passengers will prob hear one engine shuting down so think it's ok to stand up and get there hand baggage out.
On the jet2 757 a/c both engines are shut down as normal then the ground power is attched. This is because of the height the 757 is off the ground.
Hope this helps!