BMM389EC
Correct if the # 1 bus fails the emerg lights come on. but #1 Bus can be/is powered by bus #2 via the isolation breaker and vice versa. eg when one ext power lead is used to power the aircraft on the ground.
To lose this bus you have to have multiple failures isolation breaker fail open and 2 generators fail on the same side or a complete electrical failure.
Please excuse if this is not totally correct as its from memory