Generally speaking, most ships have a battery backup which energizes only crtical components until the generators come on. For about 25 to 30 years, automatic transfer switchgear equip have been able to bring generators online within 11 seconds (standard spec). The batteries are, more or less, seamless for critical systems like lighting (lighting on the bridge is minimal... almost dark as the displays provide enough light for crew movement and make the displays sharper) and nav on the bridge, computers, comm, whatever. Very little emergency lighting in the rest of the ship is powered by the batteries... mostly exit lights and very little "guide" lighting.

As for a requirement for a second engine, have you got a link for that? The only specs I have found, as I posted earlier, say : "Engine: Wartsila, 12-cylinder" There is no plural. ONE engine as far as I can search on the net. I have been in engine rooms on twin prop ice breaker class ferries and both engines were in the same room.

It's gonna be a long while before the story is told.